As long as the GSP reaches the higher end of the salinity level, then it will live a long healthy life. They live in full marine water, but travel to freshwater to pump out some new GSP in freshwater. Since we don't breed them, its better to keep their water condition stable with no fluctuation with SG
You have many options for the tank if the GSP goes full marine, like putting in live rocks and other marine fishes (not recommended). At the higher end of the brackish levels, you have less options. Either way, just make sure your GSP gets water that is close to marine water as an adult.