More specifically:
- The final deliverable must consist of a GB or GBC ROM file.
- GBC features are allowed (entries may be GBC only)
- SGB features are allowed (but don't rely on them)
- You can use RGBDS (ASM), GBDK (C), ZGB (C), GB Studio, GBSDK(C+RGBDS ASM) or anything that produces a valid ROM.