I have a gripmaster (it's honestly not enough resistance) and will say that it'd work better if you did grip-type pull ups or lifted weights using your fingers to hold them. Really do read the link I gave, it's really good as it targets those muscles more effectively than using a grip tool like a Gripmaster. That thing is a tiny portion of it.
Also limit your grip training to have 2 day breaks in-between, you can compress the nerves in your hands and it will damage you with arthritis and other complications. Avoid any and all sharp pains, if you're doing things right you should be feeling heat instead. Having any good workout you don't always have to be sore/in pain for it to show results, you'll feel it a day or two later.