You can install GOAT by downloading it from the Asset Library (as a plugin).
Alternatively, you can clone this entire Godot project, which also includes a short demo game. When choosing this option, Godot might show an import warning message:
This message can be ignored by clicking "Open Anyway".
In both cases, the Godot project should be reloaded after installing GOAT (Project -> Quit to Project List -> open project again).