It's always hard when you see poor fish suffering in LFS, but it's best to resist the sudden urge to rescue them and come home and research them. Even if you can put a deposit on the fish then come home and read up on their requirements, it's better than nothing. I'd avoid rescuing any fish unless I could come home and research them to make sure I can cater for their needs properly.
GSPs do start off in freshwater as Sammy has mentioned and you gradually increase their salinity so they become acclimatised to nigh on full marine salinity when they are older. If I were you, I'd have a good think about it and if it's too much work for you to maintain them then go for a freshwater puff instead. Or how about
T. biocellatus Figure 8 puffs that require very low end brackish conditions? I have two that are gorgeous and live with bublebee gobies and American flagfish

I'm about to get a lurker puffer next as opposed to a Toby marine puffer as the maintenance and outlay will be considerably less and much easier than my marine tanks are to maintain.