With Valve taking a firm stand against skin gambling sites (such as CSGOLotto that recently made headlines), Twitch has also reaffirmed its position in a blog post.
"As a reminder, per Twitch's Terms of Service, broadcasters are not permitted to stream content that breaks the terms of service or user agreements of third-parties. As such, content in which the broadcaster uses or promotes services that violate Valve's stated restrictions is prohibited on Twitch."
As most if not all CSGO skin gambling sites use methods that violates the API and user agreements on Steam, it stands to figure that streaming this sort of thing on Twitch won't be allowed. As it with Valve it remains to be seen how this is policed, and whether the gambling sites find new ways to operate.