During descent, most frequently around 3000-2000 ft on approach, I get a freeze of the sim of about 5 to 10 seconds, (during which everything freezes except sound), then it resumes to normal operation, without crashing. I don't have other stutter/freezing issues but this. Could it depend on GSX loading at the destination airport?
By default (unless you tweaked the airport visibility setting in GSX for that airport), GSX will load the airport at exactly 3.0 NM from the airport reference point. So, assuming an airport at roughly sea level, your 3000-5000 altitude range seems to be a bit farther away, indicating the problem is not GSX loading the airport from the Navdata.
In addition to that, thanks to the fact GSX runs completely externally to the sim, in the Couatl .exe, it runs in different threads, so even if GSX would hypothetically "stall", maybe waiting some data, it will NOT block the sim, because it's running in a separate thread, the only effect would be it might get the data later, but it has no ability to stall the sim.
The one and only time when GSX (INDIRECTLY) can cause a pause/stutter, is when it's creating objects. Which in MSFS only means ground vehicles at the parking spot you selected (and if you pre-select a gate before landing, they are not loaded until AFTER you land), because GSX only sends the commands to create objects to the sim, which will then proceed to create the requested objects, so it's the simulator itself which is not capable to create new objects on a separate thread and can stutter when requested to so so, that's why I said GSX is "causing" some stutter when it creates objects, but indirectly.
Opposite to FSX/P3D with SODE, GSX doesn't even create jetways anymore, they are loaded by the sim automatically.
The only thing GSX creates at an airport, at exactly 3.0 NM out, are custom VGDS added with a GSX custom profile. Those *might* cause a brief pause for the reasons explained above. But if it's not happening for you at exactly 3.0 NM, it's not GSX. And, there's an option in the airport profile to prevent their creation and only create the one you are using on your gate.
If I takeoff from airport X, fly around say for 15-20 minutes, then land back at airport X, everything is ok. If I takeoff from airport X and land at airport Y, most times this happens.
Which further confirms it's not GSX, because if you exit the airport range ( 3.0 NM by default ), GSX will go through the exact same procedure of reloading the airport Navdata, and eventually create VGDS, there's just no difference between this and after a long flight.
The difference in a complete flight, instead, is possibly exceed the maximum number of objects, which might happen during the flight and possibly accumulate, which leads to the very well known problem (in the worse situations), of entire airports disappearing on arrival.