I believe the Marines found GPS equipment in Fallujah when they were doing their raids last month.
GPS just isn't that big of a deal. I'm sure you can buy a GPS at the local Walmart.
The idea of shutting down GPS totally is really dumb. There's just way to many industries now totally dependent on it. Aviation probably has the best fall back equipment. But there are agricultural, telecommunications, surveying, and marine that would be hit hard.
Shutting down GPS totally is really out of the question, except for some kind of 9/11 repeat. Which is not very likely. Tod Beamer showed us how to deal with that. Aircraft will not likely ever be hijacked again by terrorists in the cockpits.
GPS guided "cruise missles" though are a possibility. And the best defense against that would be local jammers around DC. I've got to think they've already got them there. A couple of years ago they tested GPS jamming somewhere offshore, so the equipment exists.