Basically it uses GPS when GPS PRIMARY is displayed on the PROG page. This may help...
The FM position used to navigate is the most accurate of these three:
MIX IRS
RADIO
GPS
Imagine departing from an island in the ocean with no Navaids, and with GPS Deselected. Each FMGC will compute a MIX IRS position from the IR's and use that to navigate.
Now, as you near land you enter RAD NAV coverage, and each FMGC computes another position called the Radio Position. Provided there is a DME element the FMGC will use the Radio position to navigate. (1.22.20 p7). And it will now apply a bias to the MIX IRS position to make it approach the Radio position, thus it becomes more accurate (useful if you turn round and go back to the island).
Now enable GPS, and (1.22.20 p3) each IRS now computes a mixed IRS/GPS position called GPIRS. GPS is so accurate that to human pilots this is indistinguishable from the GPS position (subject to integrity criteria etc.). The FMGC will now use ONE of the onside/GPIRS3/offside GPIRS positions to navigate - not a mix. It will now apply a bias to the FM position to approach the GPIRS position it is using, so that it has a more accurate position available if GPS PRIMARY is lost.
And that is the important bit - you don't need to know all this, but you must know how to monitor navigation if you lose GPS PRIMARY......