It just means that your game should look like a GameBoy game. First and foremost this is called "GBJam" so you're not fooling anyone by making an FPS and slapping a 4 color palette on it. It's about working within the limitations of the hardware, or whatever approximation you can achieve with your skillset
Aaaalright, got it. Lol wasn't thinking about making an FPS for a GB jam but rather a pixel art single-screen arcade little thingy. I guess that falls into original GB themes category.
Given this example, https://gamefaqs.gamespot.com/gameboy/585702-faceball-2000, would a very primitive 3D game be acceptable?