I didn't want to step on the Montrose thread. I loved the "Best Of Gamma" disc for years, but until I got the three originals I didn't realize how different they were.
Gamma I is like mid period (late Rogers) Bad Co.; pretty good if you like that sort of thing.
Gamma II is bluesy almost metal, like a cross beteen Deep Purple & Trower. It's the best one IMO.
Gamma III is very slick, lots of synths. Almost AOR like Howe era Bad co. I like it though too.