Or maybe "distantly related" to GPS.
I've seen that output described as a Numerically Controlled Oscillator.
On the receiver I mentioned, I measured the 10 MHz output with an HP 5372A Time Interval Analyzer and got the following results:
"Measuring the period of 50M cycles shows two normal distributions, one centered at 94.0 ns (~20M readings) and
the other at 104.4 ns (~30M readings). Apparently, this load 'brackets' the 100 ns point by sending short and long cycles on
an almost one-to-one basis that average to 100ns."
Later it was pointed out that the ratio of short and long pulses would vary from unit to unit and with temperature. Note that the difference between the two periods of 10.4 ns represents a frequency of ~ 96 MHz. This is sort of in the ballpark of the 'up to 120 MHz' note regarding the clock speed that was in the spec sheet. The output might work if you were measuring frequency because the gate time of 1 to 10 seconds would be long enough to average out the jitter. Other uses might not work as well. The manufacturer didn't try to hide this behaviour and, in fact, had an application note on using a phase-locked oscillator to clean up the output.
I would still say that it was locked to GPS, but be careful - here be dragons!
Ed