Quick overview for those who are unfamiliar:
GB Studio is game maker software that creates simple adventure games in four-color monochrome Game Boy style, and actually outputs those games as functional Game Boy ROMs (for an emulator, etc.).
I've played with it briefly. It's a neat idea for creating a retro-looking game, but it's nowhere near ready for something like a mainline-style Pokémon fangame yet, nor is it meant to be at this time. Right now it's intended for "adventure games", which just means things like exploration, talking to people, solving overworld puzzles, etc.. It has no features for combat, advanced menus or any of the myriad other things even a simple old-school JRPG would need, let alone all the complexity of a creature-collecting game like Pokémon. Like, last time I checked it doesn't even support saving your progress yet? I think?
My understanding is that the dev would eventually like to support the creation of more complex games. It definitely wouldn't be tailored toward Pokémon specifically, so in order to create a proper Pokémon game you'd also need to have custom coding or scripting, or at least faaaaar more options for eventing. I think custom code is also in the cards, but probably also a long ways off. If and when those things ever happen it might be worth a look, but I wouldn't count on them in the immediate future.
That said, it seems to be perfectly fine for the kinds of games it's meant to create right now, so if you have an idea that's relatively simple and doesn't require any custom systems, it's definitely worth a look. It might be interesting to try for a more story-driven jam game, for example, or to see how far you can push its current limitations (idk, maybe you could simulate simple overworld combat by removing sprites when you collide with them while a "you have a sword" variable is set?). And it's definitely cool that it creates real ROMs, especially if you happen to have hardware you can use to play it on an actual console (assuming you still have one that works, haha). But if you want a Game Boy-style Pokémon fangame right now, you're better off either using COMBOY's Essentials mod or just outright ROM hacking an existing game.
