There are a lot of autoharp videos on YouTube as well. I've put up a few myself -- just go to YouTube and search autoharp or "autoharpist". Gm is the relative minor for the key of Bb. Like Bill D said, a 21-bar OS will cover you, but that key is at the fringe of being supported really well. My suggestion, in addition to those great suggestions above, is to get your first autoharp from someone who deals with them for a living. Not only will you get an instrument that has all the "bugs" worked out of it, but you would get expert advice on setting up the autoharp to support the keys you really need.