I'm curious to know more. Can you please elaborate or point to some articles discussing this?
http://www.ntp.org/ntpfaq/NTP-s-refclk.htm
TL;DR Most NTP networks are relying on GPS versus a high precision on-site time keeping device. Break GPS, and you break timekeeping for a wide swath of the worldwide NTP pool. But thems the breaks when you can get access to atomic clocks in space (each GPS satellite carries an atomic clock on board) just by sticking an antenna out the window.
If you require precision time for critical business operations (financial transactions, global database operations), you should be running a precision time source locally at your datacenter; for under $20 an attacker could deny you GPS timing.
Especially when using distributed databases where write priority is determined by timestamp, someone wrecking havoc with your time source could bring down the database
Just to clarify, the clocks used to demonstrate gravitational time dilation were on the order of $10,000 (or more, e.g., [0]). But one can find rubidium standards online for a few hundred dollars.
[0] http://www.ebay.com/itm/HP-5061B-Cesium-Beam-Frequency-Stand...
So the way it's supposed to work is that NTP models the error in all the above services and noticed when a source deviates. So if someone screws with the local GPS you should ignore it, and do the best you can with the remaining sources.
If you trigger NTP drift with a single source something is wrong with the setup.
I was interested in more details about why GPS is dangerous (i.e., more about "breaking GPS"). I get that a cheap jamming attack can disrupt a single NTP server/location. But it isn't obvious that that leads to widespread use of GPS being a bad thing for NTP pools. Because it would require a widespread (near-simultaneous?) jamming for several hundred physical locations to bring down a large chunk of the pool in that way. Of course, somehow corrupting the global GPS signal would be an issue, but how would that happen?