Found it. It was a post in the old discord server but as far as I can tell it hasn’t advanced at all in years.
Viewing post in Gameboy Compiler?
I've found the old discord through archive.org, it's located at https://discord.com/invite/9rAjhtr (and it's read only). I don't think this gameboy compiler has a public repository, there are only screenshots and it's probably not usable... It would probably be easier to create something on the GB with GBStudio (which is open source and easy to use)