I am using Orbx from Global Base up to North America, the pmdg 737, and almost all of the FSDT sceneries. I also use sceneries from flightbeam, flytampa, SXAD, Cielosim, Dr Zewiecki, and PacSim. This problem is not limited to any sceneries, this will happen anywhere.
As far as disconnecting, I did exit the sim after the GSX vehicles disappeared. Whenever I activate GSX, the default P3Dv4 vehicles do disappear successfully. The problem has to do with the GSX vehicles themselves disappearing whenever I select a gate while approaching into the airport from the air. Here is a step by step:
1. While on approach from the air, I select the desired airport and gate.
2. I land, and at 25 knots after landing, GSX appears at the gate ready to marshall me in.
3. After stopping at the gate, I cut the engines and set the parking brake. (Sometimes i set the parking brake first and then cut the engines, though I have not noticed whether or not this contributes to the problem).
4. I open the GSX menu, and select any desired command.
5. The desired command I selected shows up in the green banner, and all GSX vehicles disappear. All that is left are the cones.