The bottom line is simple... GPS is a legal substitute where the GPS
distance measurement matches the DME numbers... It is not legal where
the DME is offset from the VOR or airport center and the GPS database
does not correct for this... Increasingly, the DME location and/or the
GPS databases have been matched up so either the GPS or the DME gives
you the appropriate distance measurement, and offset DME's are now
definitely in the minority...

You can skip installing the DME and simply check your approach plates
to be sure you are legal (and out of the trees) when shooting a DME
approach with the GPS as your distance measuring equipment...
There is the occasional approach that I run into where the plate says
DME REQUIRED... If you are IMC then you will simply have to shoot a
different approach using your GPS... OTOH, if the fickle finger of
fate takes down your GPS (for whatever reason) you may be wishing you
had a DME on the panel... I have both GPS and DME on board and both are
in use as cross check when shooting approaches...

Denny