OP, sorry if this slightly hijacks your thread, but IMO it is worth addressing the idea of grip gadgets, seeing as they have been recommended to you.
@Real Soon : I'm sure grip gadgets have their uses, but bass playing is not one of them. Grip has no part in playing the bass. One of the first things a beginner is taught (or should be..) is to keep both hands
RELAXED . In this case developing grip strength is obsolete. We have enough strength in our fingers from a very young age to fret a string. Try it yourself. gently fret down on a string until the note rings clear. Minimal strength is required.
Stamina and agility on the other hand (pun intended

) is a different matter altogether. What needs to be developed in place of grip is quite the opposite action, i.e. the lifting of the fingers
off of the fretboard quickly and easily without them getting in each others way. If you are away from the bass and want to keep your hands in good condition
for bass playing purposes , then throw the grip gadget in the trash can and do some gentle hand/finger stretches. Another good exercise away from the bass is to get a rubber band and wrap it around all the fingers (at the finger nails) and thumb. Keeping the arm straight, expand the hand/elastic band...hold for five seconds...relax for five seconds...repeat. Do this for about five or six minutes say, twice a day. Be sure not to over do it. Ten minutes maximum at a time is all you need. You can vary the strength of the band if you wish. This type of exercise is far more beneficial (for bass playing purposes) than wasting time... and money... on grip gadgets.
Stamina is best built up gradually by simply playing the bass.