The project officially provides custom Godot builds with Goost already precompiled. Currently, only stable versions of the engine are released. If you'd like to compile Godot with Goost from source yourself, proceed to the official Goost documentation to learn how to do this.
Note: Goost is an extension to Godot and thus contains full Godot features + additional Goost-specific features some of which are in beta stage. While the Goost binaries were built using the official Godot build containers, not all of them may be properly tested as of now.
If you have any doubts regarding what you need to download, you can refer to Godot's download page for more details specific to the engine.