GPS clocks have lots of vulnerabilities and can be attacked easily. If you are in a stock exchange you to run rife is you spoofed a GPS signal. They also have limitations as to where they can be placed. The antenna length has limited reach and can't be too long either.
I have an idea for a PTP clock that I think would cost less than US$100 to make and would be unbeatable in terms of accuracy and security. The device would be placed on a PCIe board for ease of integration.