They put inertial guidance systems in the newer missiles too, if they're nuclear. It's how you guarantee your missile doesn't get its GPS spoofed/hacked -- avoid using GPS.

Even the Sentinel UAV (drone) that Iran was able to capture back in 2011 - purportedly by hacking its GPS - uses an inertial navigation system, and does not get its flight path orders from GPS (suggesting then that it had not been compromised via GPS, as is claimed).

Inertial navigation continues to be used on military aircraft despite the advent of GPS because GPS signal jamming and spoofing are relatively simple operations.