To do some pure Gameboy ambient music you could try stuff like layering arpeggios over each other to get a nice pad. Only thing is that it'd probably take up all 3 channels and leave no room for anything else. Still, you could probably make some nice tracks just taking the pad and slowly changing and evolving it.
I want to get this same sort of sound and, though I did accidentally fall on this sort of sound, I forgot about it. Now, though I kinda wanna experiment with this sound. Could make some really interesting songs with evolving soundscapes and solos and things layered in gradually. :3