Any GPS clock you are going to find in NY4 is going to be sufficiently sophisticated to check the GPS signal against its internal oscillator in order to check for validity. It can fall back to the internal oscillator if the GPS time looks bad.

The other reason this is unlikely to be a problem is that network time protocols break all the time (oops someone did a maintenance and broke sync with the master clock). Lots of real world trading code has sanity checks on the time.