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
I ended up testing 3 different Garmin GPS receivers as a Stratum 0 source for pfSense.
Less than 10 usec offset/jitter for less than $50
How to wire it up and pfSense setting here:
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I NEED to dig into your project a little closer, I need to buy/build a new clock because one of mine is dead and the other is pretty old. As we know, everything on a network needs fairly accurate time, especially with file shares. I also need to do research on building a PTP server, I have some things that can benefit from PTP to lock endpoints together, Dante audo/video can use PTP as the sync signal or generate something very similar. PTP can be used by other devices for time so having it would fill a dual role. SMTPE 2110 uses PTP to provide the same sync lock to professional audio/video equipment, not sure when/if I’ll get there.
That said, you can buy GNSS locked NTP servers on ebay for $80+, I have my eye on one that is $150 with two separate ethernet and a small display on the front. This will probably get me by until I can make my own devices with either a computer or a RasPi 5 (PTP compatible in the Pi5).