NTP GPS server is a time server that uses the protocol NTP (Network Time Protocol) receives its authoritative time source from the GPS satellite network (Global Positioning System).
Other varieties of time server exist most if not all dedicated devices use NTP as it is universally excepted as the most superlative time protocol, there are however numerous ways a network time server (whether NTP or not) can receive a time signal.
The internet is a common source for timing information although very few administrators who have a dedicated time server device would use the Internet. This is because external time sources across the Internet are the wrong side of a uses firewall and therefore pose a potential security risk not-to-mention many Internet timing sources are not very accurate and also most are stratum 2 devices and therefore further away from the original time source than a stratum 1 dedicated NTP server (stratum 0 devices are atomic clocks).
The other methods a dedicated NTP server (often called network time server) can receive a timing signal is via GPS or radio transmissions that are broadcast from national physics laboratories.
Radio signal are a good source of UTC time (coordinated universal time) as they are maintained and run by organisations such NPL (National Physical Lab. UK) or NIST (National Institute for Standards and Time – US). However, these long wave transmissions are not broadcast in every country and the signals are susceptible to local interference and topography.
On the other hand a dedicated NTP GPS server receives its time signal from the GPS satellite system (each satellite contains an atomic clock). These signals offer excellent accuracy and furthermore are available everywhere on the planet where there is a clear view of the sky.