Looks like an interesting project that'll make GB development more accessible than ever. Do you plan on stopping with the original Game Boy compatability-wise or is GBC part of the equation?
The project supports both the Classic (GB/DMG) and Colored (GBC) devices, as well as an additional Extended mode. Overall, GB and GBC are the primary focuses of GB BASIC, allowing users to set the cartridge type for each project. The extended features are tailored specifically for custom emulators, and you can check out the specific extended functionalities at https://paladin-t.github.io/kits/gbb/extensions.html. It is feasible to differentiate from GB, GBC, and extended devices within a single project by checking them in the code.