Think of it this way. A person can live a normal life with 1 arm, 1 leg or even 1 eye or ear. Is it ideal? Probably not. If you lose your other arm, leg, eye/ear it's going to cause many more problems than just losing 1.
Losing any backups in aviation is a less-than-ideal situation. That was the only point I was trying to make. OF COURSE pilots can operate a plane without GPS, my god I'd hope they could. I feel like that kind of goes without saying.
Taking off with 1 of 2 engines is a lot different than taking off with inoperative avionics. The FAA and airlines dictate what minimum equipment is required for any given aircraft to be airworthy and it’s a fairly robust document that takes into account myriad situations and flight conditions.
That's a different question.
If you're knowingly flying into GPS jamming - these jamming operations are apparently announced via NOTAMs (https://news.ycombinator.com/item?id=28833002) - you should be planning ahead even more.
A pilot is responsible for having enough fuel in case of an emergency, like the GPS unit crapping out. Anyone running out of fuel due to GPS jamming was ill prepared for other emergencies.