The concept's name is 'satellite assisted navigation' or shorter 'satellite navigation'. The name 'Global Positioning System' (GPS) is an American label for a system. Since GPS have been around for so long, the initials 'GPS' became synonymous with the concept. Kinda like 'Fedex' became synonymous with delivery that is faster than the US Postal Service.
Glonass and Beidou are also satellite assisted navigation systems in competition with GPS. Just like how the US Postal Service, Fedex, and UPS are competitors in the same market -- delivery.
INS is always used with GPS. Infact INS is a must for any fighter aircraft.
Today it is. But inertial navigation (INS) is older than GPS.
The main problem with inertial navigation system (INS) is drift...
http://en.wikipedia.org/wiki/Inertial_navigation_system
All inertial navigation systems suffer from integration drift: small errors in the measurement of acceleration and angular velocity are integrated into progressively larger errors in velocity, which are compounded into still greater errors in position. Since the new position is calculated from the previous calculated position and the measured acceleration and angular velocity, these errors accumulate roughly proportionally to the time since the initial position was input. Therefore the position must be periodically corrected by input from some other type of navigation system. The inaccuracy of a good-quality navigational system is normally less than 0.6 nautical miles per hour in position and on the order of tenths of a degree per hour in orientation. If navigation systems malfunction, they can send planes off course, such as Adam Air Flight 574.
This is purely a mechanical problem. The initial error is unrecognized by the system and since the current position is calculated from the previous position, over the long travel distance and time, the angular difference between the desired location and the actual physical location increases. The system needs a method of external correlation or correction.
One method is the astronavigation in the FB-111A and SR-71...
http://www.blackbirds.net/u2/c_bennett/bbird-03.html
Atop the aircraft behind the ACP and above tank one is the Astro Navigation System (ANS). This is the avionics package that navigates the SR-71 by the position of stars, weather day or night.
On the FB-111A, right in front of the cockpit and topside of the forward fuselage is a round glass window looking directly skyward. The stars provide that external correlation and at airliner cruising altitude, the system can use the stars during the daytime.
Today's INS is much more better than the -111 era but because there is now a greater need for accurate and precise weapons delivery, and the closer the aircraft is to the target the less need for the bomb to correct itself, satellite assisted navigation became mainstream and a necessity.