The manual pages for gropdf and grops should gave you enough information.
Looking at "fonts_x.ps" and "fonts_n.ps" shows you how the glyphs look like in the groff package hdtbl, residing in the build directory ".../contrib/hdtbl/examples" in the git repository and in Debian "/usr/share/doc/groff-base/examples/hdtbl". The Palatino fonts are on pages [26,29].
