I've got two Pi Zeros in the NTP pool, each as a GPS based stratum 1 server.
One of them is a Pi Zero connected up to the diagnostic port of one of my GPSDOs. The other is one of these Pi Zero clocks, but with a GPS breakout board hooked up as well.
Essentially, that one is a mash-up of this project and a the Pi GPS NTP project.
Since the new laser cut wood and acrylic case has a cut-out for the GPIO headers, it's easy to tack the GPS receiver on. The one downside is that you lose the serial console pins.
The next thing I'm going to do is to make a new board that's a modification of the breakout board that makes it purpose-built for the pi. It will have a 16 pin header to mate with the left-most pins on the GPIO header. It runs the GPS module from the 3.3v supply provided by the pi, and 5 volt power is applied through a blocking inductor to the antenna jack to power active antennas.
If you have one of my GPS Disciplined Oscillators, you can use this little board to connect to the diagnostic port. This allows you to make a mash-up of all three projects - the GPSDO, the clock display and the GPS NTP server.