I want just GPS location method on, but each time I restart location all forms of location are back on. How do I get Marshmallow to remember my preference of GPS only?
It was for the battery life actually. When you're in the wild (outside of WiFi and networks) (as I am frequently) you don't want the phone to waste battery looking for WiFi and networks. GPS only is not as accurate, granted, but accurate enough for when you're in the middle of nowhere.
Around home, I guess it is ok to just use all three methods, but if you want to save battery in the wild, the one GPS method is all you should use.