Linux / MacOS
Open a terminal and run:
bash <(curl -s -S -L https://git.io/install-gam)
this will download GAM, install it and start setup.
Download the MSI Installer from the GitHub Releases page. Install the MSI and you'll be prompted to setup GAM.
The GAM documentation is hosted in the GitHub Wiki
Mailing List / Discussion group
The GAM mailing list / discussion group is hosted on Google Groups. You can join the list and interact via email, or just post from the web itself.