First of all, how big is it? If it's pretty young it won't need such high levels right off the bat, the older they the more salt they need. BUT, they don't need full saltwater condtitions, just a little higher Sg of around 1.012 or a little higher should be good enough.
Puffers, at least the ones i've owned, are delicate when they are acclimated from freshwater to brackish, and like any fish they should be introduced slowly. Not only will it save your fish, but will also keep your bacteria alive.
AMS, not to come off rude because I may have read this wrong....
Did you mean taking a GSP in pure freshwater, and raising the Sg all the way up to 1.010 in a few hours with the drip method? Usually depending on the age of the puffer, they don't stand that well. I once had a baby puffer that was in freshwater, raised the Sg up 1.006 within a day and it turned black and died ( this was also when I was first starting out lol).
Oh well, I Haven't slept in like 30 some hours so hopefully I don't sound confusing >.<