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AFAIK this isn't how it works; the GPS provides a reference so the skew of the crystal can be measured as it drifts.

That's literally what "discipline a crystal oscillator" means. You measure and then correct the skew.

The crystal provides a local reference with low phase-noise, and the long-time-constant disciplining provides reasonable holdover performance.

Thanks, I wasn't familiar with the jargon or the electronics.

The original post said "might" but it seems that's exactly what cell sites use (GPSDOs).

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.

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