Your questions is a bit vague.
There are commercial GPS NTP servers or you can connect a relatively inexpensive GPS receiver to build your own stratum 1 sever. However from what little I know even though GPS time accuracy is <= 1us due to network jitter your overall accuracy will still be >= 50 us.
After some additional reading.. Using the kernel model and the PPS signal it is possible to get to an accuracy in the 3 us range.