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.
Also, some of the videos made to support the treasure hunt are available at

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.