The Airport Gates( Aircraft Parking bays on the ramp) are surveyed to a high degree of accuracy for there Lat and Long co-ordinates.
This information is then loaded and stored in the FMS and this can be either transferred into the Air data inertial reference system or entered manually.
The IRS/INS needs this to calibrate its position, but I read somewhere it really can do this without any input, so the co-ordiates are entered to get the whole process moving faster.
Also while the whole alignment process is in progress the aircraft must not be moved, Aircraft have been stuck on taxiways due to this essential procedure being forgotten.
I hope this helps!