Make your non-dominant hand into a fist, then gently stroke it with your dominant hand (i.e., the ASL sign for slow). The sign is as if you were gently stroking a little creature, so when you do the sign, the motion should be slow and your body language relaxed.
A popular ASL sign for gentle is actually both flat, open hands, palms facing down. Then make both hands gently do a sideways, patting motion, as if you're patting the heads of small children in front of you.
Gentle is a good admonition to warn your child that they need to care with something because it is fragile. This comes in useful as children get older and they don't realize their strength can easily break things. A lot of people keep using the sign even when their child gets older as an easy way to remind a child that they might be playing a little too roughly, without embarrassing the child.
Use the gentle flash card when you are trying to mold behavioral traits like kindness. Consider teaching your baby how to pet a puppy “gently”.