Having at least one navigation system that is autonomous and does not rely on a vulnerable reference seems a good idea.
Apologies before hand if I'm teaching you to suck eggs but this might be of interst to others.
Possible problems with GPS is of course why modern long range nav systems
en-route don't just use GPS as a sole reference, they'll look at GPS/Inertial/conventional radio aids and in the background whilst en-route are running fancy algorithms/confidence checks, and in extremis can veto out anything it considers to be an erronous source. Of course if at the start of play the PIC is determined to "tell" the system where it is, regardless of where it really is, HAL has a problem................
I think most of us here mentioning the use of GPS are talking about it's use before flight on the ramp, as providing the reference position for aligning the inertial system. If you do that (as we do in our SOPs) there should still be a credibility check to ensure the GPS position isn't wildly in error, for whatever reason ( spoofing, lack of coverage, etc). That usually means in part selecting the electronic MAP display and comparing the real world seen out of the window with the virtual one. For example checking a 16R departure via Dunes from SYD if all was OK I'd at least expect to see the departure runway and the Dunes waypoint displayed on the
map display a short range scale.
Apologies again if etc....