Regarding Gwen herself, especially with her flippant attitude regarding the lives of unimportant "background" characters. Is she right about them and a good person trying to use what she knows to survive in a new and dangerous world? Or is she becoming a villain whose knowledge of comic book tropes has given her the delusional belief that she is, at worst, an anti-hero?
The comic itself notes that the latter interpretation is perfectly valid, and even makes it a central plot point from issue 18 onward.
After seeing Gwen's old life in issues 16 and 17, people have been wondering how her life got so bad, why she dropped out of school, and why she struggles. A lot of people have come to the conclusion that Gwen suffered from an undiagnosed mental disorder or illness, like depression. Considering her issues are largely absent in the main Marvel universe, and a throwaway line establishes that her lactose intolerance was cured when coming into the comic book world, another popular theory is that her depression was also removed.
Author's Saving Throw: The initial basis for Gwenpool - Gwen Stacy BUT AS DEADPOOL - wasn't well received as a series concept, but the reinterpretation of her as a different flavor of Meta Guy to Deadpool with no relation to Gwen Stacy beyond their first name and hair color was better received, as was her debut story in Howard the Duck.
Base-Breaking Character: Fan initial reactions to Gwen were mixed, with the usual calls of pandering and selling out being slung. Some saw her as just Marvel trying to create an Alternate Company Equivalent of Harley Quinn, but after her first issue, many stated that they were surprised out how well written and thoughtful the comic was and are optimistic for the future. Also, the reason Gwenpool was created at all was because of how popular the Gwenpool variant cover was with Cosplayers, so there was a demand.
Epileptic Trees: How Gwen got into the Marvel universe isn't revealed, but there are a lot of fan theories about it. The first one is that she's physically ill and sees herself as a burden to her parents (due to her canon acceptance of her being erased from her parents' memories) and that her adventures are just escapist daydreams where she can avoid the trappings of reality. The other, much darker theory is that she's part of a Make a Wish Foundation deal, and was a dying girl who was written into the comics.
It seemed these theories were jossed in issue 16 when Gwen's home life was revealed, but then issue 20 strongly implies that even the back story of the fake dimension was fake and that Gwen no longer remembers her old life.
Oh worse, the Nightmare Fuel entry about the issue implies that she could be somehow you, the reader. That's This Loser Is You on some brand new levels.
Christopher Hastings, in the final issue, effectively proclaims that it really was you the reader, who brought her in, but not the way you thought — she came to the Marvel Universe because people loved her since that gag cover on Deadpool's Secret Secret Wars all those years ago.
Hard to point one since all the characters in Gwenpool are incredibly fun but Batroc and The Terrible Eye are probably the most beloved by fans.
The comic itself is this to the "All-New, All-Different"/"Legacy" era of Marvel, almost everyone who can let pass the fact that the sole reason the comic exists is for an Ascended Meme finds out that is pretty entertaining.
Evil Is Sexy: Evil Gwenpool is certainly more curvy than her good counterpart.
"Funny Aneurysm" Moment: A lot of humour, particularly from Gwen's guest appearances in other stories, comes from Gwen's constant screw-ups, or how much people dislike her. Not so funny now that we learn her backstory, and see that she was a cynical NEET who lost all hope in herself, saw herself as a major screw-up, and had a strained relationship with her parents. The same things that were jokes earlier are actually things that hurt her enough to make her abandon her family and go to another world.
Fridge Brilliance: Why did Ronnie design Gwen's suit with no legs? As she said herself the shade of pink used is both very specific and unpopular so she likely couldn't make legs with the supply she had.
Becomes double-brilliance when Ronnie, likely pestered by Gwen, gets a much larger supply of pink fabric. Then Gwen comes into possession of an army of mooks and commissions Ronnie to dress them all in Gwenpool uniforms. Ronnie runs out of pink again and nobody gets leg coverings.
Germans Love David Hasselhoff: Gwen's quite popular in Japan due to her cuteness, the book's artwork, and because breaking the fourth wall is a popular concept over there. Gwen's even gotten exclusive merchandise overseas. Ironically, Gurihiru, the main artists of the book, once said that they couldn't find a lot of work in Japan because their style was too "Western."
Growing the Beard: Gwen's first appearances in Howard the Duck shows the character as little more than a shallow self-insert character, facing no consequences for her actions and acting like an Alternate Company Equivalent to Harley Quinn. But when she is forced to work for MODOK, who out of universe is regarded as a joke by casual fans but can be really powerful in-universe, she starts to learn the consequences of being the protagonist of a superhero comic and her story gains a lot more depth.
After Gwen has a small breakdown in issue 20, she goes to Terrible Eyes for comfort and calls the latter "her best friend". After that, she makes amends with Miles Morales and promises him that she will be a full-on hero from now on.
The final issue. Lost and ready to cross the Despair Event Horizon, she's confronted by an older Gwenpool who helps her through this moment by explaining it's not the end at all, giving her a few extra adventures with other characters, including teaming up with Spider-Gwen, Spider-Man, Silk and Miles Morales and befriending a little girl who was a fan of hers... but saving her from a bunch of bullies with Fing Fang Foom.
Jerkass Woobie: Gwen. Painfully lonely, trapped in a world not her own, surviving on pure luck, so it's not hard feeling sorry for her... unfortunately this brings out her worst sociopathic tendencies, so she would probably kill you for it.
Les Yay: Several of her interactions with other female superheroes have her coming off like a lovestruck teen trying to be The Casanova. Especially with Kamala Khan.
Memetic Loser: Even among her fans, she's a huge Butt-Monkey. This largely stems from her being nowhere near as strong as many of the heroes she meets. The fact that she's often drawn with a flat butt has also reached Memetic Mutation, along with parodies about her trying (and failing) to be sexy.
"Is fucking Bendis writing this?" explanation This comes from an exchange in the Rocket Raccoon and Groot: Civil War II tie-in, where an exasperated Gwen wonders why Kitty Pryde is in the area. This panel is often used as a reaction image to insult Bendis books.
"We never left the Comic"◊. explanation The final page of issue #18 gained popularity, due to how exploitable it is. The most common edits involve Gwen tearing the page and showing another version of herself.
Teddy's attempts to bring Gwen back home to her family and his Only Sane Man treatment of being in a comic book has led to people comparing him to Marche, usually with an image of him delivering Marche's own memetic line about escapism being unhealthy.
When the book was announced to be ending at the same time as America (2017), tons of people made jokes about Gwen sacrificing herself to kill that comic.
Never Live It Down: Gwen was never a selfie-obsessed millennial, and took one selfie during her own story, in order to confirm a kill for her client. The earliest use of her taking a selfie and hashtagging only came from a Howard the Duck cover. Still, nearly all other Marvel properties (such as several figures, crossovers, guest appearances and video games) show Gwen taking selfies all the time.
For some, others stopped reading West Coast Avengers after issue 2, since they found her completely different to her original series version and more similar to her issues in Howard the Duck, main complaints is that Gwenpool is surrounded by heroes and she barely reacts to it, and then she starts having an active romantic relationship.
The backhands Gwen has given to Marvel, like complaining about Bendis' writing or mentioning how Secret Wars would probably mess up any current plot certainly help.
Rescued from the Scrappy Heap: A small example. Gwen was a majorly divisive, as many people outside of her fanbase found her annoying, or thought she was just a gimmick character. After issue 16 showed her backstory and revealed how relatable she is to many people, she slowly became a fan favorite. It helps that the next issue had her deal with real life issues and struggles, with made her more liked.
Rooting for the Empire: A lot of fans found themselves rooting for the evil future Gwen and wanted to see her succeed in turning our Gwen evil. The evil Gwen is fun, powerful, sexy, and is offering Gwen near-ultimate power without (permanently) hurting anyone. While our Gwen ultimately refuses to torture people she cares about, fans thought that her becoming this awesome character would be a great twist of events. This type of reaction generally comes from people who missed that Gwen supposed to be a self insert character and horrifies those who didn't.
What an Idiot!: Gwen casually trying to shoot Miles Morales' classmate on the basis of him being "a bad guy", despite it being incredibly obvious that he would object.
The Woobie: Teddy, Gwen's brother from the real world. He travels with her to the Marvel Universe, but while Gwen becomes a mercenary, Teddy spends his time scared out of his wits. He comes close to dying on multiple occasions, can't hold a job because he has no ID, becomes homeless, and is forced to do warehouse work for a villain. When he finally does see his sister again, he's horrified to see her ruthlessly gun down a group of men. And then learns that in the future she becomes even worse with, even her friends wanting to escape from her due to her ability to seemingly manipulate reality itself. All of his effort to bring his sister back to the real world winds up being for naught, and it's implied that he's failed to save her multiple times. And then he learns that the "real world" is, in fact, just another comic book world. Even worse, the final issue reveals that when he disappeared again he ended up in Hell.