gnus-propfont.el by OliverScholz displays articles in Gnus in a proportional font. It is possible to define faces on a per-group basis. It is possible to define rules to exclude certain parts of an article from being “fontificated”. So that – for example – code snipplets are displayed in the default font and not in a proportional font.

After putting the file into your LoadPath, add the following to your ~/.gnus file:

    (require 'gnus-propfont)
    (add-hook 'gnus-article-prepare-hook 'gpf-add-faces)