You first have to use a program like AFCAD to edit the airport that you wish to land at. Some of the smaller airports in FS9 will not have that many parking spaces let alone a gate. There are different size spaces for each type of aircraft. The best way to make sure one will work for the plane you want is to pick the biggest space. By doing this you should have no problems
Than like stated in the other post you have to edit the aircraft cfg file. The best way to do this is to assign it to a airline like Delta but be careful the aircraft that you want to park here has to be defined as a Delta aircraft otherwise no go.
I is a little tricky but if you take your time you can figure it out. Just compare your aircraft to a default aircraft
This is a sample of an aircraft cfg file
atc_airline=Alaska
atc_parking_types=GATE, RAMP, CARGO
atc_parking_code=ASA
The line that READS atc_airline=Alaska is the line that defines this aircraft as an Alaska Airline
you can see the different parking types used here this is the line you will need for gate parking
Notice the parking code is ASA this is short for Alaska Airlines
so to define you plane as a Delta aircraft
you would have to use the following lines
atc_airline=Delta
atc_parking code=DEL
atc_parking types=GATE
This should do the trick