"We're going to see this through to the bitter end. Or... lager end. "
The leader of the group, who assembles his school friends in order to complete a failed bar crawl from their youth, thus setting the events of the movie in motion. While charismatic and persuasive, he's hedonistic and irresponsible, and unlike the rest of his friends never outgrew his teenage mentality.
The Alcoholic: Everyone else sees how harmful his binge drinking is, although he sees it all as a bit of fun and seems to be in denial. Gary is still in the teenage mindset of alcohol making you an adult. He's stopped drinking by the end of the film.
The Stoner: Gary also does a lot of drugs, or at least did when he was younger.
The Alleged Car: "The Beast", which he bought from Peter some 20 years ago. The other guys are not entirely impressed that he's still kept it, particularly since it's still failing in spite of all the repairs that Gary's made to it. By the end of the film, it's been destroyed.
Attention Deficit... Ooh, Shiny!: has a short attention span and a "selective memory", forgets details of his friends' lives, is dismissive of danger and generally seems very out of it. This is partly because he's a Jerkass and partly because he's an addict, and a suicidal one at that.
Badass: He takes on many blanks and becomes a professional wandering badass by the end of the film.
Badass Longcoat: He wears a longcoat to invoke this. As part of the overall Deconstruction, however, while it might have looked badass as a teenager, what have obviously been several years of wear-and-tear between then and the events of the movie instead means that when he wears it he looks more like the shabby, unkempt alcoholic tramp he more or less is than the untouchable king of cool he imagines himself to be. The longcoat actually ends up looking pretty cool in the final scene.
Big "WHAT?!": When Andy orders tap water instead of a beer at the first pub.
Blood Knight: Seems to enjoy fighting the blanks the most, egging Steven and Andy on at certain points. He's also the most aggressive other than Andy when dealing with the blanks. He gets into a rumble at the very end of the film, seemingly enjoying himself too.
Death Seeker: His motivation for going to The Hole in the Wall, telling his friends to forget about him and save themselves.
Deconstruction: He's basically one of the Man Child characters of Spaced, Shaun of the Dead etc. taken to the point where his hedonistic refusal to grow up a bit has long since stopped being charming and has become a bit pathetic and disturbing.
Despair Speech: His monologue to Andy close to the end of the movie, when he states that all his hopes of a better life after high school never came to fruition, reveals the real reason for the bar crawl (i.e. a Bungled Suicide attempt), and says that he's jealous of Andy for having a perfect life.
Destructive Saviour: He saves humanity from the Network's domination, but plunges Earth back into the Middle Ages in the process.
Determinator: He's really single-minded when it comes to completing the pub crawl, managing to drink even during a fight.
Disco Dan: Mentally, it's clear that Gary obviously prefers the eighties and nineties to the modern day.
Driven to Suicide: Gary attempted to commit suicide at some point before the film due to his disillusionment over how the high school pub crawl seemed to be the only high point in his life.
Dye Hard: In-Universe. He's been regularly dying his hair black for over twenty years, but his stubble and eyebrows give his natural hair color away.
Fingerless Gloves: Wears sleeves over his hands that keep his fingers bare to achieve this effect. Conveniently enough, they also serve to hide the bandages from his suicide attempt.
He's inconsiderate and selfish, and his friends find him irritating and pitiful. In a very atypical Earn Your Happy Ending, he becomes a steadfast, loyal friend to the young robot versions of his gang, thus finding his purpose in life.
Whenever the five friends actually relax into the pub crawl and have a good time, which is what Gary wanted, he's the one that makes things awkward again. Andy rightly points out that the other four Musketeers are just there to give Gary another excuse to get drunk and that he needs to start growing up.
Foreshadowing: When telling Andy of his plan to begin the crawl again, 12 pints from 12 pubs, Andy points out how ludicrously unhealthy this is. It's possible Gary intends to kill himself with alcohol poisoning.
Inferiority Superiority Complex: He acts like the cock-of-the-walk to hide the fact that he's immensely unhappy - suicidally so - about how he feels his life has gone nowhere but downhill since the Musketeers' first attempt at the Golden Mile at the end of High School.
Literal-Minded: Gary takes most arguments and subtle insults at face value.
Made of Iron: He's survived years of substance abuse, at least one drug overdose, a car crash, a suicide attempt, an alien invasion and the eponymous end of the world, among other things.
Man Child: He is an idiot who still acts and dresses as he did as a teenager.
Manipulative Bastard: watch him cajole all of his friends into returning to Newton Haven and then, after things go bad, into continuing the pub crawl.
Mistaken for Special Guest: The reason that The Network listens to Gary in the first place. Gary goes around yelling about how he's "Gary King, of the humans". The Network hears it as "Gary, King of the humans".
No, Except Yes: One of Gary's traits. For example, tells Sam he doesn't want to have sex with her in the stall (but if she's wants to, he's okay with that.) When explaining he wants to talk to Reverend Green about Blanks, he insists he won't be buying drugs. Well, he might also buy drugs, too
Rebuilt Pedestal: At least partially—they do reach an understanding, but part ways afterwards.
Sad Clown: Gary is obviously not entirely happy, although he does his best to show otherwise.
Scars Are Forever: Has one from when he fell off the stage in his younger years and dislocated his arm. He opts not to show them when asked for proof that he's him, as that would reveal his bandages.
School Idol: As far as he believes, anyway, although he was shown to be reasonably popular with the girls back in the day.
Sesquipedalian Loquaciousness: "Tonight, we will be partaking of a liquid repast as we wind our way up the golden mile commencing with an inaugural tankard in the First Post, then on to the Old Familiar..."
In the beginning sequence. His later experiences in the town and the way his friends react to him would seem to suggest that he wasn't quite the untouchably cool king-of-the-town beloved by all he remembers himself to be, or at least that what was cool in the past isn't now. Apparently, he's become a meme among his friends ("doing a Gary"), and it isn't a positive one. Gary's less-than-pleased to learn of it.
Gary: Is that a thing?
Steven:(sighing heavily) It's always been a thing.
During Gary's flashback, he talks about Mr. Shepherd being one of the teachers he really liked—however, after he leaves, Mr. Shepherd looks sort of disgusted with Gary.
Deadpan Snarker: Any time Gary makes an over the top statement, count on Andy to snark back.
Foil: He's a serious, responsible, married teetotaler - that is, a complete opposite of Gary.
Genius Bruiser: Rugby player and also a lawyer. Even when hammered, he's one of the few to notice Oliver's birth mark re-appearing, and he also provides some very good arguments that the Network cannot refute, as any good lawyer should.
Happily Married: Or so he claims. He's actually been having major difficulties as of late, although they get resolved by the apocalypse.
Gary's friend, the only one to show any enthusiasm about the bar crawl. A fitness fanatic with a successful career in architectural consulting, he has a failed marriage behind him. Also, he's been in love with Sam Chamberlain since their teenage years.
I Will Only Slow You Down: Offers this when he's caught by the Blanks. Subverted: he actually manages to slip them, making him a rare example of someone who will Only Slow You Down managing to eventually catch up again.
The Lancer: Despite being friends, he's a bit of a rival to Gary. (His last name, "Prince" to Gary's "King", also hints at this status within the group).
May-December Romance: at the start of the movie, he's in a relationship with a 26-year-old fitness instructor. He dumps her after the events of the movie and gets together with Sam instead.
Nice Guy: A very decent guy, Steven is very pleased to see all his friends again, and is a little more polite and tolerant towards Gary (though he has his limits).
The Rival: To Gary, for Sam's affections (among other things).
Distinguishing Mark: The 6-shaped birthmark on his forehead responsible for his nickname, which he has had removed via laser surgery. It returns on his double, which the gang notices.
Faux Affably Evil: When he gets replaced by a Blank, his doppelganger is eerily polite and extremely friendly. It's one of the things that gives it away, since the real Oliver was friendly but more reserved.
Heel-Face Turn: A weird case. Oliver was a Face the whole time, but his Blank doppelganger wasn't until The Network left.
"If you think I'm going to have sex with you in the ladies, you're crazy!"
Oliver's younger sister, who crosses paths with the group during their pub crawl. She's the object of affection for both Gary and Steven; the former slept with her during their teenage years.
All Girls Want Bad Boys: She'd been there as a teenager, but by this point she's outgrown that mentality, probably because of being left behind that night. While she admits that she doesn't exactly look back on their fling without fondness, nowadays she just can't look at Gary as boyfriend material anymore.
Action Survivor: Compared to the others, she's less of a straight-up fighter. However, she can make it through some fights through sheer determination.
Badass: As soon as she sees evidence of what's going on, she immediately steps up and gets involved in the fights.
Badass Driver: Drives impressively when rescuing the guys at the end of the film.
Nice Girl: Extremely polite and in spite of the ribbing between her and her brother, they get on well. She's not, however, soft.
While she is rightly disgusted and disturbed by Gary's behaviour towards her (on top of not being appreciative of being left behind that night) she does care about him enough to try and get through to him how unhealthy his mindset is on more than one occasion.
Single Girl Seeks Most Popular Guy: The reason for her fling with Gary all those years ago (and her pining for Adrian Keane). However, she's outgrown this mentality, because...
The group's old teacher, who they have a lot of respect for.
Affably Evil: Genuinely polite and friendly to the gang in the present as a Blank, buying them all a round of drinks and asking that they address him as "Guy" instead of "Mister Shepherd". Even after he gets his head smashed by Andy, he's willing to buy another round and have another polite chat.
What Happened to the Mouse?: No details as to what happened to him come up in the epilogue. It's not even that easy to see him in the crowd of Blanks that leave the ruins of Newton Haven. Granted, he didn't have a huge role until after he got replaced, but still...
Affably Evil/Faux Affably Evil: Genuinely somewhere between the two. It wants a genuinely better universe and will reward those who volunteer generously. On the other hand, refuse and your arse is grass. Literally. You'll be mulched into compost.
Big Bad: The biggest threat to other worlds and the Earth, and the reason that Newton Haven, as well as most of the world, has changed so much.
Humans Are the Real Monsters: Its belief is that humans are the least civilised race in the galaxy. The guys concede that they aren't perfect.
I Am Legion: Speaks through the Blanks at times, such as commandeering Blank!Shepard, and speaks through Oliver, Peter, Trevor, Shane and the Marmalade Sandwich (all at the same time.)
Not So Stoic: Eventually, the Network is screaming at Gary and Andy.
Omniscient Morality License: Deconstructed. It serves mainly to piss humans off about him being high and mighty, to the point where Gary and the vast majority of humanity eventually tell him to buzz off and accept a societal breakdown so he can leave us alone.
Villainous Breakdown: Gets increasingly jittery and less "high and mighty"-sounding as Gary and company continue to refute his points, until he drearily says "Fuck it." and leaves.
Well-Intentioned Extremist: Wants galactic peace and unison, with humanity joining the wider stage, good. Will oppress those that don't comply, bad. Willing to go as far as to mulch non-compliant people? Very bad.