First, let's catch up on what the show brings us before we see the movie.
In "The Void", there's cheerleader tryouts and they fail.
Then Mr. Small pops up behind Gumball and Darwin. He notices that the cheerleaders are failing because they're obviously missing something.
Gumball and Darwin notice suspicious pictures in the yearbook.
They notice that Molly is missing and Janice, Mr. Small's girlfriend. Darwin holds up a piece of paper that shows him and Gumball in front of Molly's treehouse.
They go into a "void" full of the world's mistakes.
They find Molly and get her out of the void.
In the episode "The Oracle", it is revealed that Banana Barbara can actually paint the future. At the end of the episode, she paints the Wattersons in the void.
In "The Inquisition", Superintendent Evil turns Elmore into humans and then live action people, or at least he tries to.
He was Rob, and he had a good reason for all this. But before he could explain, he gets sucked into the void, and that's when the series ends.
So I think the movie will be about the Wattersons in the void and Rob avoiding them to go into the void.