
When GPS integrity event is detected .GNSS revert to DME-DME or VOR-DME even VOR- VOR.
Navaids in Europe are enough to cover DME-DME or VOR- DME(-100NM) ...which comply to RNAV 5 specification.
One way to detect spoofing, if you are equipped with 2 FMS/GNSS ,is to put one FMS in GPS mode and the other one in NAV(aids) mode.