"The whole spectrum" for GPS FHSS being a relatively tiny frequency range. So still trivial to jam.

There are physical restrictions on transmitter aerials which make really wide FHSS impossibly difficult. Especially from a satellite. So GPS still transmits on the same standard 5 frequencies, with some special codings.

The antijamming features work a little differently. If you know a random code sequence you can use various digital noise reduction techniques to improve the SNR and help punch a signal through noise.

But if you blanket blast the transmitter frequencies and their associated components with noise, you're still going to have an effective jammer. It will just have a smaller effective range.

In the military context, violence is an option. Powerful radio transmitters are vulnerable to anti-radiation missiles: https://en.wikipedia.org/wiki/Anti-radiation_missile In a free fire zone a GPS jammer would have a lifespan measured in minutes. A hypothetical conflict with a near-peer would involve a lot of jam and counter-jam drones destroying each other, until both sides run out of equipment.

A more interesting attack would be jamming the M code and then spoofing the data on the civilian band, which is one hypothesis for what happened to the RQ-170 that Iran soft-ish landed: https://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incid...