I'm trying to figure out what the correct .Xdefaults resource is which
controls certain a certain class of gnus faces that I don't like.
Yes, I know that I can do this through customize, but this font
appears so often and in so many different faces that this would be
seemingly unmanageable.

The font I'm trying to change is what is used in the following faces
for example:
 
  Gnus Emphasis Italic
  Gnus Signature Face
  Gnus Summary Low Read Face
  Bbdb Company

In this screenshot from a customize session, "Gnus Header Newsgroups
Face" and "Gnus Header Content Face" show the font I don't like and
between them "Gnus Header Name Face" shows my default font.