can i customize a parking position and save it, that the next time i'll go there everything will be there? thanks!
That's not what the customize parking is meant for. By customizing a parking, you will set the parking dimensions, type of objects accepted and the starting positions for vehicles.
But in GSX, vehicles will always be placed on the selected parking WHEN you call GSX service. So, the normal way of operation is to select a parking as soon as you land, so all vehicles will be already in place when you taxi there.
If you are starting a flight parked on a gate, vehicles will appear when you select any GSX service.