From reading this I would say yes.
http://www.aquatic-hobbyist.com/profiles/brackish/gsp.html
"I suggest keeping GSPs at low-end BW when juvenile <2", (in a specific gravity, or SG of 1.005-08), at 2-4", medium BW (SG 1.010-15) and adult >4" SW (SG 1.018-22)."
"GSPs are probably the most commonly available puffers in the aquarium market today. Mostly sold as freshwater fish, they are really a brackish water species. Born in FW, they migrate through the estuaries (streams) between FW lakes and the ocean, to live out their adult lives in saltwater. Although you may hear these fish do fine in FW, they will grow larger, have brighter coloration, suffer less disease and live longer in higher salinities. It is our duty as fish keepers to keep our friends in the best possible conditions for their health and happiness.
Puffers must be introduced into a fully cycled tank. Please do not cycle your tank with puffers in it, or any other fish for that matter. Fishless cycling is quick and doesn't do harm to a living thing. You can also "instant cycle" with Bio-Spira. You need to add your fish immediately after adding these live bacteria to your tank. Aragonite or crushed coral substrates are used to help maintain a stable alkaline pH of around 8. I suggest keeping GSPs at low-end BW when juvenile <2", (in a specific gravity, or SG of 1.005-08), at 2-4", medium BW (SG 1.010-15) and adult >4" SW (SG 1.018-22). You must use marine salt. You will need a hydrometer to measure the salinity. It takes a cup of salt/5 gal of water to raise the SG .005. If the puffer you purchase is in FW at the store, then you should start out your tank in FW and raise the SG .002/week, until you reach the desired SG. This is so you don't destroy the good nitrifying bacteria as you add salt. SW bacteria are different than FW, so you need time for the SW bacteria to develop as the FW bacteria dies off slowly. If the store keeps their puffers in BW (congratulations, you've found a store that really cares about their fish), cycle your tank at whatever SG the puffers are living in at the store. If you already have an established BW tank and are buying a puffer kept in FW at the store, you need to acclimate it very slowly. Whenever I change a fish over from FW-BW, BW-SW, or visa-versa, I use a drip system. I put the fish in a bucket below the tank I will be moving it into, covered by water from the tank the puffers were living in, about 1" over its head. I tie a knot in an air hose until it drips enough water into the bucket to raise or lower the SG in the bucket .001/hour."
