Vessbot is correct, in terms of basic theory and generic mechanical internal platforms, INS 101, you do not need to enter present position or use GPS to align north/south...a basic platform will probably simply use a process called gyro compassing.
At the equator if the platform correctly levelled (first part of the align process) and by luck happens to be directional aligned correctly and then if the platform is uncaged the north/south axis will not sense a tendency to “tip” as the earth rotates under it, but the east/west will (the east end will have a tendency rise relative to the local horizontal, the western end will have a tendency to dip.). However if it is not aligned correctly then the north/south ends will have a tendency to rise/dip....so the platform is motored/slew/rotated until the north/south axis is stable, and bingo, you are aligned...no GPS or magnetic info required. As V has said it is slightly more complex away from the equator but the same logic applies. You can use an analogous process (using fancy maths) to align strap down platforms, through the motoring/slewing in that case will be virtual....ultimately of course you do need to enter start position at some point to get the aligned platform to produce meaning output...
This used to be accompanied by lots of flashing lights (and cursing from the rear seat on some aircraft)...nowadays it’s press “on” and enter GPS PPOS....