Ballistic missiles since the early 60s had inertial internal guiding systems, fully independent of any satellites. Navigatinal Satellites would be shot down first in case of an all-out nuclear war.
I believe the UGM-27 was the first to be targeted via something like GPS, but it was only used for determining launch site, and I think that stayed the norm until the late 80's when the GPS systems could be shrunk and hardened for high-g operation. So yes, most Ballistic missiles maintained an inertial guidance system for many years after GPS was launched. But it was the requirement to know accurate position of mobile launch sites (in order to preprogram the missile's flight computers with appropriate inertial target solution) that motivated the system in the first place.
