I’ve seen headcanons I’ve liked about Mabel being trans, and headcanons I’ve liked about Dipper being the twin who’s trans, and both have made sense because identical twins are biologically incapable of being different sexes. Fraternal twins are, of course, but not identical twins – and so far as the fandom is aware, all twins bearing the name Pines are, in fact, identical.
Which is to say that while I don’t know whether I like Dipper being trans or Mabel being trans better, I really like the idea of people in the Gravity Falls universe realizing that one of these supposedly “identical” twins must be trans and the other must still identify with the gender they were assigned at birth. I just love imagining the reactions the twins have when said people start asking rude, idiotic questions. “So, which one of you is the real/original twin?” “Are you actually supposed to be two girls, or two boys?” “Which one of you is faking your gender and which isn’t?” Etc. and so on, infinitely spiraling down the well of idiocy and transphobia as they go.
Mabel wiggles her arms dramatically and does a “spooky” voice, crab-walking in front of Dipper for extra effect. “No~bo~dy kno~w~s,” she extends each vowel for, like, three minutes to make sure this person feels uncomfortable for as long as possible. She’s very theatrical about the whole thing. In fact, I wouldn’t be surprised if she sometimes tells them to wait a second and runs off to get a sweater labeled “Mystery Twins” and then stands in front of them explaining that this wasn’t the sweater’s original purpose, but maybe now they get the idea that (and she extends the words even longer this time, just for good measure) “No~bo~dy kno~w~s! It’s a my~s~tery~!”
Dipper gets extremely defensive, taking his angry stance and practically shouting at the person to “Mind your own business, you creep!” in the most determined voice he can manage. He grabs Mabel by the hand and tells her they’re leaving, since they have better things to do than listen to ignorant creeps asking stupid questions. Mabel lets him, sticking her tongue out at the person who started interrogating them, trying to get the last word in as the twins exit the scene.
The person idly wonders whether Dipper getting defensive was because he’s the trans twin and he’s sensitive about it, or because he’s just that protective of Mabel. They never do figure it out, though; since Mabel and Dipper only talk about gender with people they actually trust – and even then, they try not to make a big deal about who’s trans and who isn’t. They’re just… they’re just Dipper and Mabel, and that should be good enough. And for almost everyone (except the occasional idiot who alerts the twins to their presence with painfully transphobic questions), it is.