In that case the problem is that customize is using font.el to do
the
conversion and that does not recognize 'gtk as a device type and
silently fails.
Applying this fix makes it work
Obviously this code is already present for mswindows and used to work so
thats not it.
There is something else weird going on with the face frobbing. I am
going to investigate.
This is all gtk code so it seems unlikely that this is the cause.
andy