Just some places that we find are good for hardware info/trades/purchasing.
INSTRUCTABLES - Tutorials website from Autodesk. There are plenty of Game Boy based tutorials for everything you might need.
GAMEBOY COLLECTORS UK - Not the biggest Facebook group for Game Boys, but my personal favourite. Straightforward selling rules and friendly members.
CHIPMUSIC.ORG - For the Chiptune crowd - If you want to make music with your Game Boy, the Nintendo Handhelds forum is the place to look for answers to many of your questions. Search before creating a thread!