To be honest, unless you know why the sign says this, it's not completely clear. Is it warning that your GPS will turn off? Is there EM interference around the area? Should I follow the road unless I've got GPS active?

I suspect it would take me a moment to process this into some action.

Yt doesn't help that there's a substantial number of "the best way to X is Y" (language varies) type signs that are not there because it's the best way, but because a semi truck once got stuck or because there's a turn involved that's terrible at rush hour or some other reason that boils down to the "wrong" way being the better option except for some edge cases.

A lot of people confuse "GPS" with "moving map software". I had a neighbor once say "My work's address is wrong on the GPS". There is likely nothing wrong with the GPS signal. It's just that the map data in the area may be known to be incorrect.