What is GMod BETA?

Months ago, I saw these pics of GMod uploaded by Garry. I said “Ooh how cool! I can’t wait to try this out!” as I thought it was pics of a new update that was released. Sadly it wasn’t a release. I googled it a bit and I found out that there were members and garry using Gmod BETA.

I would love more information about this. Also, how do you participate in this? Does it cost anything? And one last question: When do they aim for GMod 13 to be released for the public? Thank you.