[dfox]

The last paragraph is what the FB's timecard (and essentially any other rubidium based frequency normal) does. You use the GNSS frequency output as a reference for PLL that does fine tuning (on the order of tens of ppm) of the rubidium oscilator (or even normal crystal oscilator). The reference oscilator modules usually have some kond of input for this kind of fine tuning (in xtal case this analog input typically controls biasing of varactor that introduces parasitic capacitance to the crystal, in rubidium case it is usually done digitally and it changes the division factor in the feedback path of internal PLL).

Edit: and for that matter even ntpd/chrony does this tuning in software and can compensate for imprecise and/or slowly wandering host clock generator (obviously it cannot compensate for abrubt changes and intentionally does not even try to).