Proper-HW game is doing it in hard-mode 😂 , I've done both with a proper rom (with sdcc  and my own crt0,etc) and a JS variant for another compo where I was kinda short on time. 

Awesome! Hopefully I'll manage to finish something in time (decided to go 2 days ago but as usual I have too ambitious game-ideas 🤣 )

This is the reverse though, a GBJam entry being entered in the GB Compo.

I think as long as it was made in the timeframe, and can run on real hardware, there shouldn't be a problem. But I'm not a judge, so don't take my word for it.

The big difference between GBJAM and GBCOMPO is that Compo games must actually run on a gameboy/gameboycolor,  where GBJam games just have to look like gameboy games.