What he said, squared. About the only thing I can think of that you can
do
with a DME that you can't with a terminal-certified GPS is use it to fly
an
ILS-DME

No reason you can't use your GPS to fly an ILS/DME. There's no
requirement for the DME shack to be in the database. My GX55 is just
such a unit. Terminal/enroute only. You navigate ILS/DME's using a
distance from the outer marker.