I just had this happen when using a PMDG 747X at KDFW. When GSX was initiated, all the necessary equipment appeared and everything worked until I called for pushback, at which point I was told pushback wasn't available at that parking spot. So I cancelled GSX and configured the spot for the necessary right turn on pushback (while also fixing a couple of other things like the unnecessary airstairs at the rear door). I then relaunched GSX and again called for pushback. Although the message on the green information bar told me the tow truck was moving, I could clearly see that it wasn't. After waiting a minute or so, I cancelled GSX again. Although I already had the latest version, I used the pull-down menu to reinstall GSX. When I tried again, the tow truck moved as expected and the pushback was completed perfectly. So I would suggest that you try a simple reinstallation of GSX -- you don't even need to leave FSX to do it.
