Before a gate can be seen in GSX, it must be seen by the sim first so yes, you must add the gate with an AFCAD editor, I suggest using Airport Design Editor ( ADE ), which is free and constantly updated.
The procedure is likely different if the scenery is default, or it's a 3rd party, but here is not really the place to learn about editing airports. The only thing you can be sure of, if you have a proper AFCAD for any airport, GSX WILL use it, provided you place it on an higher level in the Scenery Library, higher than the airport you are trying to modify.
The most important thing is, to be used by GSX, an airport should be self-contained, meaning it should contain at least one parking, a runway, and some taxiway layout. If your modified AFCAD satisfies this conditions, GSX can use it. What you cannot do, is create a small "enhancement" AFCAD, perhaps with just the missing parking. GSX won't use it in this case, since it will only use ONE AFCAD at a time. GSX is not able to take data from multiple AFCADs at once and "merge" them in memory to use all the info, that's why the AFCAD must be self-contained.
