The actual solvent for Gemtac according to the manufacturers is dry cleaning fluid - perchloroethylene (perc). You can buy it on ebay and amazon, but it's fairly nasty stuff! I don't actually have any (I must get some), but I did try having something with Gemtac residue drycleaned and all the glue was gone when it came back, so I think it does work.
I've only had limited success with acetone on Gemtac, but it's great for hotfix stones. Dab a bit on with a Q-tip or cotton pad and they fall right off. With Gemtac, I find the stones themselves eventually come off with acetone, but it's easy to damage the fabric trying to get the rest of the glue off. Not so much colour changes as abrasion from the scraping. I'd use it on dry fabric, or you will dilute it.
Acetone would certainly be enough to let you put a seam where stones were, but for major changes in the stoning, it might be better to try to get some perc.