You enter a/c position through the FMC CDU (Control Display Unit), though having said that if you really wanted you could enter your position through the INS ISDU (Inertial System Display Unit)
You can enter the gate no if you want, though most people tend to enter airport ref position. Most gate nos are not in our data base, the airports are, and with one person programing the FMC during pre-flight there is less room for error ! Of course the beauty of having GPS on the a/c as well, is that it starts updating your position before you move from the gate.
Sorry to steal your thunder Bobby I was bored.
Rgds