In the Gulfstream one pilot flies "blue" (FMS) while the other one flies "green" (NAVAIDS- VOR/ILS, etc). If RAIM goes bad, or FMS doesn't switch over to approach mode (0.3 RNP), then you can't fly an FMS approach. Having the co-pilot in "green" lets you continue the approach by just handing the jet over to him, while you get your side set up in "green".

The Honeywell FMS's that Gulfstream uses look for GPS first, then DME/DME, VOR/DME, and finally inertial nav (IRS). If GPS signal is lost, the FMS will look for multiple DME signals to triangulate.

If you lose GPS overwater and out of range of NAVAIDS, then you are stuck w/IRS's which are good for 6.2 hours before they drift too far out of tolerances.