I have a GSP, he is about 4.5 in now so a 30 would be a min tank size for one. Also they do become full marine when adults so that is something you may want to think about when planning your set up. GSP's also need shells of some type to eat, they have beaks that will overgrow if they dont. Good luck with your fish!
Definitely going to need a 29+, no tankmates. He will need increasing salinity as he matures, make sure that it is gradually done. Also, find out what salinity he currently is in. If he's in freshwater, then gradually ease him into brackish. Good luck!