I'm pretty sure that it's a requirement for people who make games with GB Studio to not bother with the extension. Seems like most of them don't bother to name them anything other than "game.gb" D:
It was made without the Color Only mode, might've been that I was using an old version of GBS.
There's nothing physically stopping someone from playing it on a DMG, but... why? Why would anyone opt for that when varied palettes are a big part of the WoH art style?