That's normal, the parking editor it's only available if an airport hasn't been customized already and, since all FSDT airports are, and the two other trial airports (EDDM and LIML) are as well, you can't use the parking editor when GSX is in Trial.
If you want to try it anyway, without purchasing it first, for example at default EDDM, remove the following file:
FSX\fsdreamteam\Couatl\GSX\airports\EDDM.PYE
It's best if you do this with EDDM or LIML Trial airports, don't remove any of the FSDT files, because they are heavily customized, and you'll lose everything by removing their customization file.
