Hello! Does Ground Services X work with flightbeam airports?
GSX works with every FSX airport. On FSDT and Flightbeam, it works for free. Meaning, it works in full mode, even if you don't purchase it. Purchasing it, will allow to use it everywhere.
Also, are there any popular aircrafts or airports gsx doesn't work with?
Not, it works everywhere, except very borderline and unusual cases, such as Aerosoft Madeira, where the airport altitude in the AFCAD is not the one the airport really is ( to simulate the specific elevated ground ) and, I think there's just one case of an airport from OrbX that has different parts of the airport on different altitudes. But it's a couple of special case airports, out of many thousands available.