I can think of one difference:
INS computes position parting from sensed acceleration in the north-south and east-west axis. It integrates it to obtain GS and track, and integrate those to obtain distance flown and position.
GPS computes position directly, and then can calculate with time to obtain GS and track.
GPS is good for static position, but in motion, INS (or IRS, too) are better in the short term. That is why FMS systems with GPS use a position which is the GPS position "enhanced" with the ability of the IRS to calculate change in position. or something like that...