Hey, I know that thing!
Yeah, it's C-based. It builds games off of this repo:
https://github.com/cppchriscpp/gameboy-bootstrapThe C code is a bit of an abomination, if I'm honest. I got fed up with GBDK early on, and didn't bother making the code very readable.
Was a fun project a while back... I admittedly don't do much with it now as it hasn't gained much of a following. I'm a bit more focused on NES stuff right now. (Though I'd be willing to jump back to this at some point if someone finds it useful!)
I don't really monitor this forum or I would've commented sooner. Thanks for the interest.