Stylus ships with a temporary version of styl.lang for GtkSourceView, based off Yanekk's work on scss.lang.
This is a start and provides a basic language spec for GtkSourceView editors such as gedit.
Installation Steps
Download styl.lang to your local language-specs folder:
mkdir -p ~/.local/share/gtksourceview-2.0/language-specs/ && wget https://raw.github.com/LearnBoost/stylus/master/editors/gedit/styl.lang -O ~/.local/share/gtksourceview-2.0/language-specs/styl.lang
Update the MIME database and enjoy Stylus syntax in gedit!
cd ~/.local/share
update-mime-database mime
This is much more enjoyable than having gedit recognize your .styl files as Apache Confs!
Have a sweet tooth? Add more icing to gedit with gedit-icing: https://github.com/niftylettuce/gedit-icing