In order to enable syntax highlighting for reST in gedit v3 simply copy the file
reST.lang
to the language specs files of GtkSourceView (which seems to manage
highlighting for all langages in gedit):
/usr/share/gtksourceview-3.0/language-specs/
Then you can switch the highlighting manually in the footer bar of gedit.
You can find all language specs officially supported by GtkSourceView, including the latest version of rest.lang, in the official GNOME repository. Additional language definitions may be available from the GNOME Wiki.