Warning: TAWOG spoilers
Ok so, as a huge TAWOG fan and multigender person who relates a lot to Gumball, I have no idea how I didn’t see this sooner. Hear me out on this, ok? Gumball has always shown that he has no problem expressing his feminine side and even enjoys being a girl and feels happy as a girl (as shown in The Dress and when he pretends to be Muriel). Now, a binary, non-dysphoric transfem may show indifference towards being a boy, but Gumball shows happiness being a boy.
That alone is pretty much evidence but I also wanted to explain how I see Zac and Muriel. Both Zac and Muriel had their own episode, both ended with Gumball learning a lesson and realizing that being Zac *or* Muriel wasn’t something that would make him happy (in this case, Zac and Muriel represent his male and female sides respectively). Multigender people often get told to choose one gender, but Gumball couldn’t. Being just Zac or just Muriel wouldn’t make him happy, which is why he is Gumball instead.
I have no idea if this makes any sense because it’s late at night and I’m tired but it’s just something I thought of. It’s just a silly little headcannon I thought of so obviously don’t take it too seriously but it makes me happy to think that Gumball is multigender because we don’t get much rep.