Dick wrote...
Can a GPS with compass, heading & groundspeed capability be used legally
in
lieu of a separate compass and airspeed instruments on an experimental
homebuilt airplane? Or at least in lieu of the separate compass?

Yes to all, *if* the inspector signs it off. Whether or
not it's a smart thing to do will probably be the subject
of some debate. I vote no, mostly because I think you
ought to have at least *one* onboard nav gadget that
will keep working when you lose electrical power.
Groundspeed is also practically useless in terms of
flight safety.

Dave 'wind in your hair' Hyde