I have referenced THIS PAGE before, I think it is an excellent account of someone clmbing the gripper ranks and the importance of block weight training (Brookfield). Remember closing grippers has a certain "technique", so you still have to practice with them. But the blocks work the WHOLE hand and will not only payoff in bigger gripper closes but will carryover into other aspects of grip.
Before you get duped into buying into some secret squirrel gripper training newsletter, put some time in on the above routine.
If you need to specialize and peak AFTER building a base, then throw in a cycle of KTA. That routine has a good, proven track record.