RGBDS is a free GBZ80 assembler/linker package for the Game Boy and
Game Boy Color.  RGBDS additionally contains 2 extra tools to fix
ROM headers, and convert PNG files to the Game Boy's planar tile
format.

The binaries it provides are:
rgbasm(1) (assembler)
rgblink(1) (linker)
rgbfix(1) (checksum/header fixer)
rgbgfx(1) (PNG-to-Game Boy graphics converter)