I've spent the weekend reinstalling all my FSX software on my new rig. I have a few FSDT airports as well as GSX. I was banging my head against the wall last night looking for a solution when GSX appeared to be non-responsive. I generated and read through debug logs, reinstalled using the latest version, unregistered and re-registered the product. In looking through the FSDT forums there were a few threads dealing with sound card drivers, making sure the COUALT.exe script engine was running, etc. Everything checked out but the product wasn't working.
Then it dawned on me - after installing GSX I had squelched all GSX messages because in the past they got in the way while trying to hit taxi lights on pushback.
So I turned GSX messages back on and tried CTRL + F12 and finally got a response
"GSX services not available while engines are running"
The whole time I had been testing with an aircraft which had running engines but I didn't realize they were running. Of course after I shut down the engines, and hit CTRL + F12, the GSX menu box came up right away.
Hopefully this helps someone else out in the future performing a forum search. I've made personalized notes in my software installation directory in case there is a next time.