This is no longer true.
Since the October 17th update, GSX has been updated to support the new Navdata API that came with SU10, you only need to enable it in the GSX Config panel.
The feature is still considered in Beta, because the Navdata itself is not fully complete with everything we need for GSX ( jetway data is not complete ) and in fact, it would be useful if as many users as possible would cast their vote to this suggestion to improve the Navdata API:
However, other than some side-effect about jetway recognition, GSX now works just fine with Marketplace encrypted airports.