In most airliners, INS is the primary navigator. In most modern airliners the INS position information is fed to the FMS, which then feeds the displays, flight directors, autopilot, etc.
The INS and/or the FMS can be updated by GPS or some combination of VOR/LOC/DME. When updated continuously by GPS, the GPS in effect becomes the primary navigator.
You do not see a "GPS screen" because all it does is feed information to the FMS; the GPS may or may not have an On/Off switch, or a means of inhibiting its input to the FMS. The FMS and its CDU (Control Display Unit) is the primary navigation interface for the Pilots.