GPS provides more accurate position information so EGPWS computer knows exactly where A/C is, on its inbuilt map.
IRS is subject to drift after after a long flight, and in remote areas, there may not be DMEs or VORs to update IRS position.
Having said that, some A/C only use FMC/IRS position inputs to EGPWS but then there is more chance of false warnings due to inaccuracies.