soupleaf
what i mean by this is... the entire fionna and cake universe seem to be created by Ice King. he's the one who tells the stories mostly, so are they actually accurate genderbends in your opinion??
a few notable differences are:
-prince gumball doesn't seem to be so involved in science as princess bubblegum.
-marshall lee is a lot more cruel and manipulative than marceline.
-fionna's outfit seems more like fanservice than anything since it doesn't really look all that suitable for fighting.
are these differences just because it's the Ice King who made their universe?
-Well, in the cartoons, Marshall Lee was actually told in Marceline's version of the story. So, I don't know about that. I can't say much on this because I've only had access to the show's portion of this story.
-Gumball's genderbender comes with a switch of baking in place of science. (As mentioned already)
-Fionna's outfit always seemed appropriate enough. Try telling me that bulky hat of Finn's is actually "suitable for fighting". I imagine he'd start to sweat something fierce in that - especially during the time when he had so much hair under there. Hell, even the backpack could be troublesome. (Although, I wonder, has he been wearing that backpack lately? I can't recall.) And then there's the fact that most genderbenders are emphasized, so giving Fionna a "fanservice"-like outfit does make sense. Even then, it's Ice King's universe, and he seems to like Fionna, so why wouldn't he have fanservice for himself?
I can't answer your question directly to that, since I don't really know, but it's easy to imagine that Ice King making the universe is the reasoning behind it.
My biggest problem is Cake's backstory. Why is Jake a feline instead of a female dog, even? And supposedly she was brought up by Ice Queen, so what's up with that?