Mathgate was the home of the 2014 Bitcoin Theorem Proving Treasure Hunt. As a consequence of server problems in early 2016, most of the content is no longer here.
The main exception is the final open source product: the proof checker Egal.

If you're looking for Satallax, it's here.
The mathgate website generally is on borrowed time. I don't recommend linking to it. It's likely to disappear completely at some point.