GPS is a tool that it can use either through jamming attempts or as a guidence tool for bombed up UAVs
Just not at the same time!
You can overpower GPS signals in a limited area quite easily. As those frequencies are easy to triangulate, it would also be very temporary.
My money is on those HARM-type missiles being able to lock on to the GPS frequency quite well also.
Still, all eggs in one basket is not a great idea and it would be nice to be as smart as the UK and invest in a good eLoran system which thanks to its low frequencies is virtually un-jammable. It won't give you precision approaches or targeting, but it will get you where you need to go...