Well, I just assembled it using the ROM it requires (just search for it online) and in the GEST emulator it works. I'm wondering how it uses the ROM; I suppose it uses it for some base stuff like music, but I wonder for what else it uses it.
The idea is to move away from requiring the ROM. There are portions of the ROM that are still directly included by rgbasm. Music is one task, but also other obvious things like the main game loop, items, interrupts, lots of battle code, etc.