GSPs need entertainment. Other tankmates would be great but keep an eye on your inhabitants because some puffers can be aggressive. If you don't want more puffers just give him lots of stuff to explore and change the decor slightly from time to time. If they get too bored they will pace up and down the glass and sometimes even stop eating.
I've always heard that GSPs should eventually be converted to full strength seawater within 1-2 years to maximize life expectancy, but some do choose to keep them in brackish. I've had mine about 4 months in a 75 gallon tank with a salinity of 1.012 along with a dragon goby, 2 knight gobies, 2 golden wonder killifish, 3 mollies, 2 red claw and 4 fiddler crabs. There's fake driftwood, plants, caves, and rocks but Puffy still got bored. I put another GSP in for company but it nipped the killis constantly so I had to remove it. Puffy followed it around everywhere and got depressed after I took it out so I got a figure 8. They're great buddies now and love the powerhead!