It is clear in the US:
(d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:
(2) Two-way radio communication and navigation equipment suitable for the route to be flown.
Effectively this means VOR for enroute navigation and if needed for an approach, an ILS, DME, or ADF. GPS can substitute for DME or ADF on an approach, but is only supplemental for enroute navigation unless it is a WAAS GPS, which would satisfy the requirements as being suitable for the route to be flown.
