I am convinced that, with work, they will learn to be gentle, too.
I've taught any nippers that I've had to be nice by putting my hand out with a treat in it, but closing my hand around it, with no fingers sticking out.
When they go to grab it, they can't actually take it, and any toothiness doesn't hurt the closed hand. After they don't get it, they tend to sniff, and lick the hand.
Once they have their mouths off of my hand, but are still looking at it, I turned my closed hand over, palm side up, and open it. It's in the palm area. Then, they take it.
Eventually, because teeth/grabbing doesn't get the treat, they stop trying that method.
It may not work for your pups, but if it does, it's pretty easy and less painful!
Love All Life, Thank You For Posting! :)