A good (timing) GPS receiver can easily get you within the sub-microsecond region of UTC. And yes.. stratum 0 sources do include GPS as timing source.

Conversely having an atomic clock as frequency source, doesn't automatically make you Stratum 0. You also have to synchronize it to the rest of the world and the simplest method of doing so is GPS (unless you happen to be an national laboratory).

For a Stratum 1 NTP server the local frequency source only has to provide short term stability and perhaps for GPS reception outages (holdover capability). And if you can live with a certain drift while the GPS is unavailable then crystal oscillators are more than good enough for this purpose.