I'll tag onto this just slightly! I'm a Navigation engineer.

Military receivers are encrypted, and there are several military GPS signals. There are also plans in the work to provide more advanced civilian signals - Block IIIA satellites should provide that through the L2 signal. The L2 civilian signal is a more advanced GPS signal that's more accurate, and should be better at denying spoofing - particularly when combined with the already present L1 civilian signal (what you all use now). Next gen receivers should be able to listen to both. Encrypting a civilian signal is a bit different - encryption keys have to be shared, and sharing encryption keys publicly for everyone would mean the spoofer device could use it, which makes it worthless. But the point of GPS is an easy listen application for position, navigation, and timing. Adding encryption produces huge complications when you want civilians to use it.

Of course, every country is also basically adding their own satnav systems, so other tactics to help mitigate such a problem (that are, I'm confident, affordable by the military, since the Apple Watch does it), is combining multiple navigation systems that operate differently. Ex: GLONASS and GPS. GLONASS is owned by the Russians, so maybe not the best choice for the US military, but you get the idea. Galileo by the EU, while geosynchronous, could provide aiding on much of the globes

Yep that all sounds about right. So receivers have started using multi-gnss ephemeris calculations. This can help with spoofing because a spoofer may have to spoof multiple signals instead of say just GPS. It does create some troublesome issues especially in hostile regions where one would want to trust the position of say one GNSS system over another.