The Gist support was originally written by mattmatt. It uses gist.rb and supports authentication out of the box – when you make a Gist, it’ll be tied to your GitHub account (assuming you followed the Global Git Config instructions on your account page). It also can detect and set the Gist’s language, based on your current TextMate.
Installation is simple:
cd ~/"Library/Application Support/TextMate/Bundles/" git clone git://github.com/gtcaz/github-tmbundle.git "GitHub.tmbundle" osascript -e 'tell app "TextMate" to reload bundles'
Update: Whoops, mattmatt wrote the Gist support. Updated post.