Definitely agree with cabezon. I say scrape it off the frag disc as well.. unless you want a GSP covered bump there later on. If you scrape it off now and place it on the bottom of the tank, once it's ready it will take off growing, and like Jimbo said, it will take over if you don't keep it trimmed. Since this coral is a softie, and uses a mat to spread it's polyps, it should be easy to just take an exacto knife, or coral propagation tool of a similar type, and cut through the mat to your liking. Cut at the edge where the GSP almost meets the rock, don't let it grow on the rocks, and you should be fine from having it spread all over your rocks.
I know a couple people that keep their GSP's in-line with other corals. Some coral have the ability to sting other coral, so when the GSP starts to encroach, the other coral stings it, the GSP dies, and no more growing from that section. Oh.. and just because another coral stung the GSP, doesn't mean the entire GSP colony will die off.. just the small section that was stung will die off. Then regrow. And the madness begins again.