I'm not sure how the GSG is built, but if the MP5 trigger frames will work, they have to be "clipped". Clipping has to do with cutting the SEF frame to fit on semi auto guns, if you like the contoured grip I'd suggest one of these.
http://www.robertrtg.com/store/pc/HK...S-137p3005.htm
In a semi auto gun the E and F positions on the SEF frame function the same way, semi auto. Other factors than just the trigger frame are required for FA.
Hope this helps.