GSX will of course check if your parking is occupied by an AI, and will NOT allow to select the parking if an AI has taken the parking.
Are you SURE that airplane is an AI and not, for example, a static airplane ? In that case, GSX cannot obviously detect a static plane so, it's up to the scenery developer to correctly disable parking spots taken by static because, if they are not disable, the same thing you are reporting here, will also happen to an AI, which would clash with the static.
Some developers might offer different AFCADs, depending if you use or not static planes for that scenery and, when you switch between them, you must use the "Restart and rebuild the airport cache" option in GSX, in order to be sure GSX is picking the AFCAD with static planes. This, of course, assuming there IS a different AFCAD with static to choose from.
An "AFCAD with static", means an AFCAD with parking spots assigned to static not present in the AFCAD, so nobody will use them: either the AIs or GSX.