Since there are so many different type of grips to master in the Gi, I noticed that these top 5 grip exercises (not in particular order) helped the most in regards to my grip strength for the Gi game. No Kimono or attachments are required for any of these exercises. If you have access to free weights and a pull up bar, then this alone will help a lot. Of course getting attachments like the Scramble Grip Trainer or Fat Gripz would help a lot, but if you don't have the extra funds to purchase them, then you don't have to worry about it, because these exercises will help you in your game.
1. Plate Pinches - Don't worry about going heavy here. Just worry about going for time. Endurance is more important than strength and power, because the longer that you can hold your position, the better it is to slowly apply your technique in direct fashion. This will help you a lot on holding on to the lapel and grabbing on to the wrist better, while this will help your grip strengh for NoGi as well.
2. Bar Hangs- This exercise helps a lot on endurance for grabbing on to the wrist for the kimura, arm bar, and americana from the side mount position. Bar Hangs can also assist on your grip for the Ezekiel Chokes, endurance on holding on to the label and collar. This helps a lot for NoGi too.
3. Farmers Walk- You can use dumbbells, plates, barbells, or farmers walk handles to do this exercise. This has a similar benefits on your grip as the bar hangs. The great thing about this exercise is that you get to work on other muscle groups as well from your legs, core, and upper back muscles.
Don't spend too much time on your grip training. 2 to 3 times a week for 5 to 10 minutes should be sufficient enough, since you do use a lot of grip during BJJ training alone. I highly suggest doing grip training after your BJJ training, on days that you don't train BJJ, or several hours before you train BJJ. That way, you don't fry your grip before class.