It's hard to say exactly without reading it, but generally in a crossover fanfiction like 'gumball the shell crossover fanfiction', it could be a creative exploration of how the worlds collide. For example, if 'Shell' has a certain type of magic system and 'Gumball' has its wacky family dynamics, the fanfiction might blend these. The characters from Gumball, like Gumball himself or Darwin, could find themselves in the 'Shell' world facing challenges that are a mix of both universes' typical problems.
In 'gumball the shell crossover fanfiction', it might be about the Gumball characters being transported to the Shell world or vice versa. This could lead to all sorts of interesting scenarios. The art style and humor of Gumball might be combined with the unique elements of Shell. There could be comical misunderstandings between the characters of the two, or they could team up to overcome a common enemy that exists due to the crossover. It could also explore how the different cultures or rules in each world affect the characters when they are in the other world.