Note
You might want to use gedit-markdown instead.
Here is very basic markdown syntax highlighting and a snippets file for Gedit (gtksourceview-1.0). I'm not the best regexper around, so it is only a quick (working) solution.
There is a default syntax style, but it's far from optimal for markdown. I recommend you to change this in gedit's preferences.
- Markdown syntax file (v3 Tuesday, 19 June 2007) -- install into ~/.gnome2/gtksourceview-1.0/language-specs/
- Markdown snippets file -- install into ~/.gnome2/gedit/snippets/
There is now a MarkdownSupport page at the gnome wiki.