Hi!
The folder in github is not a sample project. It's a set of files you would copy into an existing project. It includes some example scenes but the "spine-unity" folder in itself is not a sample project.
Spine-Unity is also comprised of two runtimes. The base spine-csharp, and spine-unity. If you are pulling from github, you would need to include both.
The gizmos meta file shouldn't be there anymore. Thanks for reporting that.
Anyway, it's easier for you to install or try out the spine-unity runtime by using the unitypackage download.
Spine Unity Download
Create a new blank project and import the latest .unitypackage into it.