Is GMAT still available? Where? How? (NASA's General Mission Analysis Tool)

In this comment below the question Visualization of non-Keplerian orbits for solar sails, NASA's open source General Mission Analysis Tool is suggested. Sounds great!

My google search found this blog from 2012 and this big PDF Users Guide also dated 2012, both on SourceForge. But the links I was aiming for didn't help much.

The link https://gmat.gsfc.nasa.gov/ only gives me a "your connection is not private" error (shown below) even when I delete the s and change it back to http:// , and http://opensource.gsfc.nasa.gov/projects/GMAT/index.php seems to be a dead end.

Where can I find this software? Is it something I have to compile on my computer, or is it hosted somewhere?