As explained many times on the forum, GSX vehicles are not aware of the external world and objects around them. They only know the airport AFCAD and the airplane doors locations.
If a GSX vehicle runs into another GSX vehicle, you can customize the parking to have a different starting position for the vehicle that is being overrun, it might fix the issue.
Sometimes, the problem might be the AFCAD too, lacking a node to calculate a proper approach or, the opposite, having a node too close to the parking. Not possible to check if you don't indicate precisely:
- Which airport you used. If the scenery is not default, clearly indicate the scenery and possibly the AFCAD in use
- The exact gate you were parked at.
Note that, if you got the Passenger Bus, but the gate had a jetway, maybe a fake/static jetway made by a 3rd party developer that doesn't supply the scenery with proper FSX jetways, you will have to tell GSX there's a jetway there, using the "Parking has a jetway" option in the scenery customization dialog. This way, no passenger bus will appear.
And, if your parking doesn't have a jetway, but is so close to a terminal building that it would make sense having the passengers taking the bus, you can disable just the Bus.