1st, GBRs have some rather uncommon tank requirements, so this in itself will limit you (ie. water temp, quality, etc.).
2nd, GBRs are only aggressive towards their own species, or when spawning and they will attack any fish entering their territory.
3rd, once past the tank requirements, the next hardest thing to overcome is finding a mating pair or you will always have sparring fish...(what size tank)
You may have more difficulty in finding other cichlids that work with GBRs than the other way around in trying to see what GBRs will tolerate.