If gentle is specifically about treatment of the pet, the "pet-gently" sign is entirely appropriate.
The sign for "soft" is to hold your hands palm-up in front of you, very slightly cupped, then bring them down gently while drawing your fingertips together. Do this twice.
I'm trying to think of when I use this -- I sometimes use both together ("pet-gently", "be gentle"). I guess anything non-petting related -- "that's delicate -- be gentle with it", that kind of thing.