I prefer
Ernest to
Gideon, but I also prefer
Ernst to
Ernest.
Theodora and
Ernest are excellent together.
Ernest Arnold sounds fine; so does
Ernie Arnold.
Gideon Arnold doesn't seem to flow as well. Gid and Giddy don't bother me particularly.
Where I live, it seems (though I've got no way of researching!) that people are tending to use the full name rather than even the most obvious nn -
Robert, for instance. The youngest
Bob I know is in his 40s.
Elizabeth is the exception I suppose, but it's not in the same league as
Gideon.