Yes, you need to do a Live Update now, we added this for MSFS, but if you update it now, it's working in FSX/P3D as well, the variable is called:
FSDT_GSX_MODEL_CACHE_READY
It's called "model" cache, because GSX has two caches, one for airports, and another for its own models and they are both recreated when the option to restart with cache regeneration is chosen, so if your goal is to know if GSX is ready, it's when the model cache regeneration is completed.
