Originaly part of the installation requirement with GPS was that the GPS would automatically get kicked off if an ILS frequency was selected. One would have to be careful selecting nav frequencies on the VHF to avoid disconnecting the GPS.
Part of your verification when flying the ILS, however,is that not only are you tuning to and listening to the ILS, but that you've got your cockpit set up to fly it...which includes verifying the source of your nav signal to cockpit instrumentation.