Here is the source code of the full (commercial) version of the game.
You can buy the game from here (available as ROM-only and Physical Game Boy cartridges):
https://drludos.itch.io/gb-corp
To compile this source code, you'll need GBDK2020 (built on version 4.06) and do:
#Use GBDK2020 to compile the main game program
lcc -Wa-l -Wl-m -Wl-j -c -o gbcorp.o gbcorp.c
#Use GBDK2020 to compile the GBT Player output (C code from mod file)
lcc -Wa-l -Wl-m -Wl-j -c -o music.o music.c
#Use GBDK2020 to assemble the GBT Player engine (ASM code)
sdasgb -plosgff -I"libc" -c -o gbt_player.o gbt_player.s
sdasgb -plosgff -I"libc" -c -o gbt_player_bank1.o gbt_player_bank1.s
#And finally, assemble the ROM using GBDK2020 (set it as SGB compatible (-ys) and CGB compatible (-yc))
lcc -Wl-yt0x1B -Wl-yo2 -Wl-ya1 -Wm-yn"GB-Corp" -Wm-ys -Wm-yc -o gbcorp.gb gbcorp.o music.o gbt_player.o gbt_player_bank1.o