Longer term there is danger that adversaries will simply shoot down GPS satellites instead of jamming them. Current anti-satellite weapons can only hit targets in LEO but I'm sure that Russia and China are researching ways to target higher orbits.
There is talk of reviving eLoran which is enhancement of Loran long-range navigation. But countries keep shutting down transmitters because of lack of use.
Finally, it might be possible to use other satellite constellations for navigation. In particular, it sounds like Iridium provides timing information. In the worst, could add something the big constellations like Starlink.
> In a preemptive strike, Red launched a massive salvo of cruise missiles that overwhelmed the Blue forces' electronic sensors and destroyed sixteen warships (...)
> Van Riper used motorcycle messengers to transmit orders to front-line troops and World-War-II-style light signals to launch airplanes without radio communications.
> The FNO course takes naval officers who have passed the navy's first-stage navigation qualification, Preliminary Navigating Officer, and qualifies those people to navigate larger ships of frigate/destroyer size. It also teaches them to do it in the absence of GPS, falling back on age-old skills that would have been familiar to sailors in the early 20th century.
https://www.npr.org/2016/02/22/467210492/u-s-navy-brings-bac...
https://en.wikipedia.org/wiki/USS_Fitzgerald_and_MV_ACX_Crys...
https://en.wikipedia.org/wiki/USS_John_S._McCain_and_Alnic_M...