At the 224th annual Grampies Award Ceremony, all Cartoon Network characters participate for their chance to win a Grampie only to find out it was rigged for Uncle Grandpa to win every time.
In the epilogue "The Grampies", Uncle Grandpa hosts an award show, featuring cameos from other Cartoon Network showssuch as Dexter's Laboratory, Johnny Bravo, The Powerpuff Girls, Samurai Jack, The Grim Adventures of Billy and Mandy, Foster's Home for Imaginary Friends, Adventure Time, Regular Show, Ben 10, The Amazing World of Gumball, Steven Universe, and Clarence. Including speaking parts from Natalie Palamides, John DiMaggio, Zach Callison, and Roger Craig Smith reprising their roles as Buttercup, Jake, Steven Universe, and Belson Noles, respectively.
During the show, there are some other Cartoon Network characters from other shows seen in the audience, Mordecai and Rigby from Regular Show, Mac from Foster's Home For Imaginary Friends, Billy from The Grim Adventures of Billy and Mandy, Clarence and his friends from Clarence, Finn and Jake from Adventure Time, Johnny Bravo from Johnny Bravo, Blossom, Bubbles, Buttercup from The Powerpuff Girls, Amethyst from Steven Universe, Dexter and Dee Dee from Dexter's Laboratory, and Samurai Jack from Samurai Jack.
Sumo Sumouski from Clarence is shown with eyelashes, although he doesn't have visible eyelashes in his home series.
Something similar goes to Gumball Watterson from The Amazing World of Gumball, as he is in his season 2 design in this short, though in the show, his design is updated.
Uncle Grandpa winning all the time is an obvious middle finger to Nickelodeon for having Spongebob win Favorite Cartoon every year.
Although there was a vast array of Cartoon Network characters that participated, not all of the major characters from all the shows on Cartoon Network were featured.