Green Spotted Puffers (AKA Wal-Mart Puffers) are ONLY brackish as juveniles. As adults, they more to full salt water. You can go ahead and acclimate them to salt water a little bit at a time. Test the water they come in, then raise the salinity by a little bit at a time over the course of a month or so until it reaches full saltwater conditions. My GSPs thrived in full salt water; they were more active and grew faster under those conditions.
However, I do NOT recommend keeping GSPs in any tank except a predator tank. They are generally VERY aggressive fish, even moreso than most puffers I've kept. You will not be able to keep a cleanup crew and you will not be able to keep small, docile fish like chromis. They will end up as puffer food. GSPs will also fin-nip other aggressive fish, even ones many times their size.
Additionally, GSPs get to be six inches or more long, and they're very active instead of spending a lot of time hovering like many puffers do, so they need a pretty large tank.
In summary: I love GSPs to death...but I don't recommend them, unless you're willing to make a purely aggro/predator FOWLR tank. And don't buy the Wal-Mart ones, if you decide to get one. Find them at LFS. They're almost guaranteed to be healthier.