[insert masturbation joke here]
First of all, the gripping muscles for nogi are different than for gi. You to be able to close your fist strong for the gi. For nogi is the wrist and forearm, its about how strong you can bend your wrist. Understand this when trying to work out. Don't just squeeze something, its all about that wrist and forearm.
I agree with Nathan, manual labor is the way to go. You get work done AND train grip strength. I am from the country, manual labor is a part of the lifestyle, I had above avg grip strength my whole life, never needed to work it out. If you YouTube wrist and forearm workouts, I'm sure there's 100's of legit workouts. Lol, i bet there's 100's of shitty ones too..welcome to YouTube.