Adored by the Network: Cartoon Network gave this a huge buildup: a "sneak peek" six days before the premiere (which also aired on Boomerang and streamed on the website), a countdown bug on-screen the day of the premiere, and at least one rerun a day for the entire week afterward.
The Latin American version of Cartoon Network brings this trope Up to Eleven. They went to such extent to dedicate a whole day to the show. They also use them to announce the new stuff of the month and air reruns three times a day, everyday, and they often air clips of this show during the commercial breaks for no reason at all.
Cartoon Network also put a lot of faith into the show by renewing a second season a week before the premier, though this could have been done as the show spends a long time in production. Cartoon Network later renewed the show for a fourth and fifth season before the third season had even started airing.
The Danza: The character Teri (the hypochondriac paper bear) is played by Teresa Gallagher. Given Teri is just a supporting character and Gallagher also voices one of the main cast, this was probably a coincidence.
Dawson Casting: With the exception of the Watterson children (though Anais' voice actornote Kyla Rae Kowalewski is a teenager while Gumball and Darwin's voice actorsnote both the past and current ones were/are 11 and 12 years old), the kid characters are played by adults (mostly Teresa Gallagher, Stefan Ashton Frank, Alix Wilton Regan, Adam Long, Max Cazier, Jessica McDonald, Kerry Shale, and, in the case of Sussie the chin puppet, Aurelie Charbonniernote in season one and Fergus Craignote from season two to the present)
Fake American: Most of the cast besides the voice actors for Gumball, Darwin, and Anais are Brits doing American Accents. Technically averted with Dan Russell (the voice of Richard, Mr. Fitzgerald, Coach, and Tina in the first season), and Sandra Dickinson (the voice of Granny Jojo, Hector the giant's mother, the pink cupcake lady, the orange woman with the blue boy, and the first voice actor for Miss Simian), who are American actors based in the UK.
Flashback with the Other Darrin: A flashback in "The Name" redubs a scene from the first season episode "The Curse" with Gumball's third season voice actor. The same goes for a flashback with Darwin in "The Extras" to the second season episode "The Sweaters"..
Granny JoJo and Miss Simian (first season only) is Sandra Searles Dickson (Trillian)
The first voice of Darwin voiced Gossamer the monster on The Looney Tunes Show and was in the Tyler Perry movie, I Can Do Bad All By Myself (Kwesi Boakye). His replacement voice (Terrell Ransom, Jr.) was on such American shows as Days of Our Lives, The Middle, and Children's Hospital.
In Brazil and Poland, Nicole's voice actress is the same one who voices Wonder Woman.
Interestingly, the Brazilian Nicole and Richard have mentored Rachel Berry; Richard used to be Rachel's high school teacher, whereas Nicole is her current dance instructor. note Nicole didn't date Finn Hudson.
Non-Singing Voice: Unlike other musical numbers Darwin is in (barring ones where it's obviously supposed to be a different voice like "No More Mr Nice Guy!"), "What He Thinks About Us!" in "The Words" is performed by a different voice actor than his usual one.
Gumball was voiced by Nicky Jones (aka Chowder) in the early reel version of the show (which is named "Gumball").
In the first season, Mr. Small, Tobias, Banana Joe, and Alan were all played by Rupert Degas while Ms. Simian was voiced by Sandra Searles Dickinson. From season two on, Mr. Small is now voiced by Adam Long while Hugo Harold-Harrison voices Ms. Simian, Tobias, Banana Joe, and Alan (though Dickinson still voices Granny Jojo and other female characters).
Tina also changed from Dan Russell to Stefan Ashton Frank, even though Russell continues to play all his other roles (including Richard).
Both Gumball and Darwin were recast in season three from Logan Grove and Kwesi Boakye to Jacob Hopkins and Terrell Ransom, Jr. respectively due to their original voice actors hitting puberty. The season three premiere "The Kids" is both a farewell to Grove and Boakye and an introduction to Hopkins and Ransom, Jr.
In season 1, Sussie (the chin puppet) was voiced by the creator's girlfriend Aurelie Charbonnier but in season 2 she is replaced by British comedian Fergus Craig. Charbonnier, however, still plays the physical puppet, just with Craig's voice dubbed over.
Almost all voices were recast in the Hungarian version after season 1, save Gumball, Darwin, Nicole, Anais and a few side characters. Season 3 changed even more voices, including Gumball's and Nicole's. Now Darwin, Anais, Banana Joe, and Penny are the only characters with their original voices.
Prop Recycling: A lot of the characters are rejected advertising mascots Ben Bocquelet had in his portfolio.
Short Run In Peru: Several episodes have aired first in Brazil (Portuguese dub), France, Spain, Central and South America (still in English), and Australia.
Talking to Himself: Most of the dozen plus named students in Elmore Junior High are voiced by a few people, so this happens a lot. In particular, every non-adult female save for Anais, Tina, Sussie, and (from the second season onward) Carmen is voiced by either Teresa Gallagher or Jessica McDonald.
In an interview (page 127), Ben Bocquelet revealed that the show was originally pitched as an [adult swim] show and about rejected cartoon characters in a remedial school. Cartoon Network found the idea to be too depressing and Bocquelet changed it focus more on Gumball and his family and make it more like a daytime Cartoon Network show (though, like most Cartoon Network shows, it has the dodgy humor of an Adult Swim show, despite carrying a TV-Y7 rating).
An "Early Reel" was made for the show as a pilot. The most noticeable differences are the fact Darwin was animated in CGI and Gumball had a square head, though other characters looked different too. Teri was also a boy, there.
Sarah the Ice Cream's name was originally going to be Dolly◊.
Four episodes were scrapped in the first season: two were called "The Mom" and "The Pizza", the other two weren't named yet. Season two also had an episode planned called "The Rex" which was cut because it would have gone over budget. However, the premise for "The Mom" may or may not have been used as the premise for "The Mothers" and "The Pizza" will end up being a season three episode. "The Rex," which would have fully introduced Tina's father and have been a parody/homage to Jurassic Park, has, so far, been scrapped.
Write What You Know: The turtle drying out in "The Puppy" when Darwin leaves him in the sun is based on how the pet turtle of the series creator died.