They also necessarily believe gravity is fake, because otherwise the entire model crumbles apart

No, they don't. Flat earthers don't believe in a big universe where planets orbits stars and shit, so this image is kind of a straw man, although it's obviously not serious.

I mean, flat eath is a dumb theory, if I can even call it that, but it is not what is in the image.

They are free to believe whatever they feel like, being unbound by logic and facts as they are.