The problem doesn't have anything to do with GSX, of course.
The problem of not being able to know *when* an AI will depart, it's due to using Ultimate Traffic, which doesn't have standard traffic schedules like every other AI traffic out there, but it generates flights based on its own database, on the fly.
This means, whenever ANY addon (and not just GSX), enquires to the sim about any AI parked plane handled by UT Live for the time of departure/arrival, or any other informations about the flight, the sim will reply with NO informations, because at that time, no flight plan has been generated for that plane yet, that's how UT Live works: it will generate a flight plan only the moment the plane will depart.
Normally, AI flight plans are stored in the Traffic.BGL file, so any addon enquiring the sim on AI status, will get all the required informations, which GSX obviously use.
We contacted Flight1, suggesting a way of communication between add-ons and UT Live, which would require they would setup some sort of API that other add-ons can call, to know in *advance* about AI flightplans, which is an information that UT Live surely already have internally, but we never heard back from them.