GSPs are very variable in terms of sociability. Mature males are likely territorial since they're the ones that guard the eggs. At all ages they may be "nippy" simply because wild fish are reported to feed, in part, on the fins of other fish. So there are at least two factors here that can make GSPs much less trustworthy than other brackish water fish.
With that said, GSPs have been kept in communities. In marine tanks they have been combined with robust damsels as well as different pufferfish species (such as Arothron spp.). Those "punchy" damsels that tend to be bullies in reef tanks, such as humbugs and blue devils, work particularly well. Clownfish seem to coexist with GSPs quite well, too. Given space, things like snappers, wrasse and triggers would be options too, but since puffers are nippy, you'd want to avoid slow-moving things like morays and lionfish, however well armed they might seem.
GSPs are somewhat reef safe. Like all pufferfish, these fish will shrimps, crabs, worms, and molluscs as potential food. On the other hand, polyps are usually left alone. Anemones wouldn't be a good idea though, simply because there's no knowing whether a GSP would understand how dangerous they can be.
As always though, "your mileage may vary". A little time spent with Google should reveal that lots of people have mixed damsels with adult GSPs, but alongside the success stories you'll get the odd tragedy.
Whilst I don't really subscribe to the idea "every puffer is different" -- they are, when all is said and done, animals of very little brain -- it probably is true that every aquarium is different. Issues such as tank space, frequency of feeding, the amount of activity inside and outside the tank likely all come into play. My SAPs and irrubesco puffers have worked in my community(-ish) tank for years without problems, but then I'm working next to them all day, and constantly feeding them, and they're mixed with pretty boisterous tankmates anyway. So they may well have enough stimulation that they don't become nippy. The same fish in a tank with slower tankmates and less attention might behave completely differently. In other words, carefully approach the idea of keeping tankmates with these large, potentially quite dangerous pufferfish.
Hope this helps, Neale
