"GSX Select Parking Facility 1-Parking" Press the "1-Parking". Another box comes up "All Parking Positions.
I just tried now EDDF, and I can see 9 Gates groups in the options from 1 to 9, and "Parking" is the last 0 option. The "Parking" area at EDDF is general aviation so, it's just normal it would say they are too small for a 747
So, your problem is not that GSX is saying the parking is too small, because it is. The problem seems that you don't see all the other Gates.
What scenery you use for EDDF ? Default or 3rd party ? Have you added an AFCAD for it ?