Can GPS be used in lieu of DME for instrument approaches? Instrument approach charts list the equipment required to fly them in their name. For this example let's say we have an ILS or LOC/DME 36R approach. This approach requires an ILS receiver for the ILS portion, or both a LOC and DME receiver for the localizer only portion.

Let's also say we're flying an airplane that has an inoperative glideslope receiver and is otherwise /G equipped with an IFR-certified and up-to-date GPS. Can the GPS be used in lieu of DME in this case?
Yes, the details are in FAA AC 90-108:


  7. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC,operators may use a suitable RNAV system in
  the following ways.  
  
  (1) Determine aircraft position relative to or distance from a VOR (see first note in subparagraph 7b), TACAN, NDB, compass locator
  (see second note in subparagraph 7b), DME fix; or a named fix defined
  by a VOR radial, TACAN course, NDB bearing, or compass locator bearing
  intersecting a VOR or Localizer (LOC) course


And all fixes, procedures etc. must be loaded from the GPS database, you can't enter any fixes yourself based on lat/long or whatever (section 9(c)(1)).