You can get Game Rig Tools from the following marketplaces. We want it to be available to everyone. So we decided to release it as a pay-what-you-want offer (where possible).
Game Rig Tools can be installed like any other Blender addon.
Download the .zip file containing the addon from the marketplace of your choice.
In Blender go to Edit > Preferences
Go to the Add-ons tab and click the Install... button
Navigate to the folder that contains the Game Rig Tools zip file, select it and click Install Add-on
With that, the addon has been installed. However, it is not activated yet.
To activate it, place a checkmark next to the Game Rig Tools addon.
You are now ready to use Game Rig Tools. A new tab called "Game Rig Tools" will appear in the Side Bar of the 3D View. All features of the addon are found there.