"The Dress" seemed to fling everything it could past the radar:
Gumball having an Imagine Spot that the two of them are married... and he's surrounded by cat/fish babies while Darwin comes home and yells, "I WANT MORE KIDS!" What really crosses the line with that scene is not only are they both male, and both children, but they're also both siblings through adoption.
Some episodes show Gumball naked, but with pixelation on his cereal-covered nothing.
Not to notice Miss Simian's saggy boobs and visible pelvis bone.
The plot for "The Third" seemed like it was talking about ménage à trois and, later, infidelity.
Darwin: Three best friends? Won't that be a bit... weird? Gumball: It's only weird if we make it weird. Do you find it weird? Darwin: Not if you don't.
Channel/Episode descriptions might not be directly tied to the studio that made the show, but one of the official episode descriptions for "The Ghost" didn't sound like it belonged to a kid's show.
"The Ghost": Carrie the Ghost often uses Gumball's body but now he has to learn how to say "no" to her, which may prove to be an extremely difficult thing to do considering Carrie normally uses his body against his will anyway.
In addition, the binge-eating scenes parody the similarly-shot music video for The Prodigy's "Smack My Bitch Up" (NSFW) whose protagonist indulges in copious drug use, drinking, and sex.
At the beginning of the episode, Gumball and Darwin both sit on their couch lifelessly flicking through TV channels and eating junk food — with the appearance of being stoned throughout the entire scene. A sample conversation:
Darwin: Can you think of any better way to spend our lives? Gumball: Dude...I can't even think. Is my heart still beating? (Darwin feels for a pulse. Six seconds pass before Gumball's heart starts beating again.)
The part where the girls are (falsely) bragging about who got the older boyfriend, the entire segment counts, but the gold is when Carrie says "mine is so old that he doesn`t have teeth."
Gumball:(while doing high dive to save Darwin from a girlfriend) On one hand, I create a diversion and save Darwin. On the other hand, I splash Penny and look like- *SMACK*
Also when Anais is first shown after the affects of the supplements, she is 6 ft tall, covered in body hair and is extremely buff. When you keep in mind that she is a 4 year old little girl, this becomes Fridge Horror.
In "The Poltergeist", where Mr. Robinson explains to Gumball and Darwin how he and his wife would argue, sometimes six times a day, sometimes in public, even once in their sleep! Anyone who is or was married (or in a similar long-term committed relationship) will recognize what he is really talking about.
A scene in "The Fight" has Gumball running through the girls shower room.
Another right after that has Gumball running across the bottom of the pool, past a one-eyed octopus wearing a diving mask and snorkel. When Tina swims past it right after, it ejects yellow 'ink' into the water.
The "baby sitter" Richard almost hired in "The Responsible" is named Albert the Pervert◊ in the concept art. Some concept art shows him as a pair of underwear, with an unsavory grin. In short, it's really disturbing that he wanted to sit on Anais.
In "The Fridge," Nicole and her family go to a paintball arena for a family outing. Outside of the arena, there's a sign◊ that lists the types of events they can cover, two of which are adult parties and stag parties!
From "The Flower"
Gumball: Wait, did you just say he was your cousin?
After Gumball and Darwin leave, Mr. Small proceeds to let out a "WOO-HOO!", like that "herbal tea" is more enjoyable than it should be...
When Darwin is raiding Joe's locker, he pulls out a jockstrap and tells Gumball to chew on it. You just know that the writers knew exactly what they were doing since neither of them address what it actually is.
On top of that, when handed the jockstrap, Gumball looks at it with absolute disgust, and then he pretends to chew on it when he obviously is not and does not want to.
"The Job" has a scene where Gumball and Darwin are delivering a pizza to a couple of pizza-headed people, which they apparently consider to be their new child. As the father is paying Gumball, he accidentally drops the box while reaching for the money he gives them, which completely horrifies the couple◊. Yes people, a children's cartoon just got away with Dead Baby Comedy.
Even worse, Darwin slips on the dead pizza baby as they run away, further smearing its tomato sauce, or as the pizza couple see it; blood, on the concrete.
In "The Bumpkin", when a bee appears Gumball hides himself from it in Nicole's skirt.
During the party montage, Gumball funnels Idaho with a soda bong.
In "The Storm", Gumball has to "inflate" Alan the balloon in the school bathroom. Just before he does it the scene skips to them then leaving the bathroom with a disgusted look on his face while Alan has an incredibly happy look on his. It should be noted the orifice that Gumball inflated Alan with, the only one his body has, he also uses to breath, eat, and excrete.
In British airings of the same episode, Gumball describes Carmen as a "bimbo" (in the US, he says the word "coward" instead)
In "The Lesson" a bear literally pops a cupcake's cherry. Prompting this dialog.
Gumball: Did you see what that bear did to that guy's cherry?
Also the rather...suggestive calender that Principal Brown planned on giving Miss Simian.
In "The Voice", Gumball takes Tobias's towel and spanks him with it in the middle of the school corridor.
In "The Castle", Gumball, Darwin and Anais disguise themselves as a pizza delivery man to get back into their house full of party animals. Being at the bottom of the disguise, the way Anais hands over the receipt probably couldn't be any more wrong.
Hot Dog Guy: "Uh... you can keep it."
In "The Internet", Gumball tries to enter the Internet by scanning himself without his clothes on. It doesn't work, but it does give Darwin a nice close up photo of Gumball's privates. Afterwards, Darwin tries to wash his eyes out.