This has been discussed and answered many times already on the forum, and it's the same for almost all our airports: all gates are flagged as GA, in order to remove the default ground vehicles that would cause a conflict (and bad fps) with scenery objects and GSX ground vehicles too.
Since you can use GSX (regardless if you have purchased it or not) at KIAH, this is not an issue, because instead of using the limited default "Go To Airport" method to position to a parking, you can use the more realistic, flexible and easy to use GSX menu, that will present you with a much understandable list of parkings, grouped by terminal and with proper names, instead of the generic default FSX names.
Use the default Go to airport to go on the runway, then open the GSX menu, select a parking, and use the "Just Warp Me there" option: it will just move your airplane there.
