In the rare event that the U.S. DOD shuts off the GPS, what's the alternative?
If the GPS is unavailable, it will be quite an impact to the aviation industry.

All airliners in-flight will experience degraded RNAV performance, but they would make it to the destination using VORs, DMEs and ILSs. For general aviation, things are not so lucky. The GPS display provides an excellent situation awareness in small aircrafts; without it, navigation reverts to old school VFR and reading charts. Most pilots will probably make it, but a few may get lost and end up in fuel exhaustion.

For airliners preparing for departure, initiating the INS is time consuming as the GPS coordinate is used as one of the startup parameters. The coordinates of the gates can be looked up in airport charts, some airports also have signs which specify the GPS location; both of these take longer time. Suddenly, pilots flying oceanic routes find out they need more planning for contingencies. Some routes may be changed to fly VOR-to-VOR. Departures around the world are going to be delayed, that is for sure.

Ground Proximity Warning Systems, which use GPS to determine the aircraft's location, are unable to use their terrain database to provide warnings to pilots, although warnings based on radar altimeters should still be available.

Basically, we are thrown back to the 80s or 90s, when GPS was not yet widely adapted. Many of those navigation aids are still in use today, so the industry can still fly, but traffic capacity will be reduced.