We wanted to support OAuth2 in Go and it was tricky to do in a TDD way, so we built Gomniauth. Today it supports Google and Github, but it's VERY easy to add providers.
It's also general enough to support beyond OAuth2, so it'll likely grow as the community uses it and decide what they need. It also plays very nicely with our Goweb package -
http://github.com/stretchr/goweb, and we've included an example app that you can copy to get going.
We'd love any feedback, issues, pull requests etc. and think this is a step towards making web development in Go a realistic option.