BACKGROUND
I picked up a used Qotom fanless PC with DE9 RS232 ports so I could finally implement my own Stratum 0 GPS NTP server.
RESULTS
< 2 usec offset/jitter typical.
Cheers! 
For those that have a little more money, there are a few Raspberry Pi version out there.
And if you have a bunch of money, there are ways to build a GPS locked PTP system out there. I haven’t had the money to buy the PTP card.
Also, since I do have some GPS locked clocks (just GPS), look for a receiver that has GNSS (GPS, GLONAS, and BAIDU), more satellites means quicker lock and better precision. I’m trying to upgrade mine right now, but it looks like the budget at work may not get me there (part of a more expensive device).
PTP would be nice, but I’m not running any high speed data acquisition systems or running a financial institution.
If I had $ to burn… https://fsmlabs.com