Same Issue here. Initial services (trucks, gantry, etc) seem to load but GSX option screen disappears after a few seconds. Restart of COUATL does not resolve issue.
A little further info. On initial spawn at Airport (in this case, EGLL), the GSX menu opened as per the attached image. If I click on the options, they seem to function, ie, the first option did indeed initiate deboarding.
When I restarted COUATL, I was no longer able to get the menu screen to stay for more than a couple of seconds. It closes.
I uninstalled and re-installed GSX with the same result.
Hope this helps