GPS can be used in lieu of VOR, DME, ADF and NDB on all localizer type approaches including when the NDB/VOR transmitters are temporarily out of service. The GPS may also be used for finding distance from a VOR, flying DME arcs, navigate to/from a VOR or hold over a VOR or DME fix. This also applies to distance/holds from/over NDB/TACAN or compass locator.
An IFR certified GPS also satisfies the requirement for DME above FL240 as per FAR 91.205(e).
The only limitation is that when the aircraft has a non-WAAS GPS and the user is planning for an alternate with a non-GPS approach that requires DME/ADF, then the aircraft must have an DME/ADF installed. This restriction does not apply to WAAS-enabled GPSes.
- GPS cannot be used for lateral naviation onthe final approach segment of a VOR or NDB/other navaid approach.
- GPS cannot be used for lateral navigation of a LOC-based approach
 AC 90-108 https://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf