The ground proximity warning system (GPWS) uses the radio altimeter to receive information about the distance from the ground. Does the GPWS system work solely based on the information it receives from the radio altimeter? or are there other instruments involved with the system? If the radio altimeter gave an incorrect reading due to something like a terrain data base error would this trigger a GPWS alert in the cockpit?
GPWS is an older technology, and it uses the radar altimeter and a few other things, but not GPS, since it was developed long before GPS existed. EGPWS, the "E" standing for "Enhanced," uses GPS and a database of terrain & obstruction heights. A system that has the latter will typically, in more expensive (and thus better equipped) aircraft at least, incorporate the former, so that you have the benefit of both -- i.e. if the GPS is unavailable, you still have warnings from the radar altimeter, or if the radar altimeter fails you still have the EGPWS functionality. That integration also handles cases when both systems would be providing an alert, so the pilots are presented with one alert rather than two at once. However, for aircraft without a radar altimeter, EGPWS by itself is valuable, so it is possible to have the one without the other, and this is common in less expensive aircraft where the cost of a radar altimeter isn't justified.
As far as inputs to a GPWS system, it will consider things like aircraft configuration, and can provide warnings when, for instance, the aircraft is descending as if on a glideslope to a runway but without the landing gear down. Depending on the implementation, pitot-static inputs might be considered as well.