Jose wrote:

The only time you can't use [GPS] is to substitute for an ADF on an NDB approach.

Any idea why that is?

Every NDB approach in the country was authorized as a GPS overlay approach when the
overlay program came into being. Since then, these authorizations have been withdrawn
for any particular runway end when a stand-alone RNAV (GPS) approach was published for
that runway end.

You cannot use GPS for any final approach segment that is not retreivable from the
database, and for good reason: no approach RAIM and no approach sensitivity.