This post details how to create a stratum-1 NTP Server using a Raspberry Pi utilizing GPS and PPS, and get time within 100 nanoseconds of real time, directly from the atomic clocks located in the GPS satellites above your head. The best part about this guide is that this will work with no internet. After initial setup, you could disconnect it from the internet and they would work. All of the other guides out there that I have found do not include the configuration necessary for this.

Source: GPS Raspberry Pi NTP Server (NetworkProfile.org)