I used a rubber band to attach mine. Since their mat is usually in odd shapes, I rubberband the ends down and then itll encrust in like a week max.
If you want to frag the GSP on a rock, break the rock with a chisel or something, and once the rock is cracked, use a razor and cut the mat in between the heads. If its just loose mat, use a razor. Injuring the polyps wont effect much. Itll grow back.
GSP seems to be a very tempermental coral, if it feels afraid it could close up for up to about 2 weeks. When I got my original frag it took a week.