If you assign a gate via the "Designated Parking Control" interface of PF3 you will be cleared to that gate by Ground after you have landed.
And TGS will guide you there reliably.
But still it can happen that you find your gate already occupied by an AI aircraft. PF3 cannot do anything about that (except for the new feature to reassign a different gate).
It is described in the PF3 Manual that a change of the AFCAD file is needed to keep the parking spot free of AI traffic.
I never liked this method too much because apart from feeling uncertain about editing the original airport file of a payware addon it implied further corrections regarding GSX and I also had consecutive problems with the CTRL-J jetways. Furthermore you cannot easily change the gate for each flight.
But there is an easier and more flexible solution for that now with the payware "Where are my aircraft" (WAMA).
The newest version of WAMA (besides its original purpose of keeping track of all your aircraft and displaying them in the sim) includes an option to define the departure and arrival gates and then have WAMA keep these gates free of AI traffic.
It only takes a few seconds to set it up and the gates can easily be defined differently for every flight. It works fine for me (using UTLive).
This is a very nice addition to the parking functionality of PF3. Now you can be sure that your gate really is reserved for you (without the need of editing an AFCAD file).
WAMA is available on SimMarket and it is not too expensive.