It would be very helpful on question chats (when they go live) to have gist 'onebox' support, to make it easy to share code.
Regarding waffles♦'s comment,
You can write multi-line messages with Shift+Enter. Moreover, you can just paste multi-line messages and they will be formatted as monospaced automatically (example: http://chat.meta.stackoverflow.com/chats/message/5211).
However, there's no code highlighting.