What are the advantages of a GPS timing reference ?
GPS is global. GPS can provide a GPS NTP Server timing solution anywhere in the world. Providing you can locate the GPS antenna with a good view of the sky, then you can obtain accurate timing anywhere in the world.
GPS is more accurate than radio time sources. GPS systems can synchronise to within nanoseconds of the correct time. Radio solutions generally synchronise to within a few milliseconds.
Provided the GPS antenna has a good view of the sky, a GPS solution can provide a ntp time server with a continuously accurate signal that is less prone to interference and maintenance periods.