Right, a normal cell site would use the 10MHz or 1 pulse per second from a GPS receiver to discipline their own oscillators. GPS could go out for a few minutes

Right, a normal cell site would use the 10MHz or 1 pulse per second from a GPS receiver to discipline their own oscillators. GPS could go out for a few minutes and everything would still work just fine. Anything past that and it's all dependent on how stable their oscillator is over time when it's free running. The WWV and WWVB atomic clock radio stations have 1Hz reference frequencies that might be stable enough to be your 1PPS reference if you designed your GPSDO to accept an external 1PPS.