Forgive me for asking a stupid question, but...
I define GD as more than just not hitting. I guess I think of it as thoughtful discipline, if that makes sense. For example, while permissive parenting is fairly gentle, I don't really think of that as GD any more than I think of super strict authoratative parenting as GD, yk? Depending on where you live it's possible that you don't know anyone who doesn't practice something that could be defined as GD.
I think another facet of GD is understanding that some behaviours aren't done on purpose to upset us parents but that they are part of a child's development. For instance, my DD tantrumming about her friend holding her toy isn't because she hates this other child. She simply doesn't understand that bunny belongs to K but M is playing with bunny now - K will have bunny back in a short while. Same thing with leaving the park before she's ready or not being able to wear a specific shirt. GD parents empathize with the child and explain what is happening and why. Non GD parents tend to think of it as acting out and will often ignore it or worse, punish it.
I hope that made some sense, we had a rough night over here.
Also, here are the guidelines for the GD forum, which give a good overview of what is considered GD at least in terms of posting guidelines. I'd be disinclined to talk too much more about "what isn't GD" because I wouldn't want to imply that I think there is an "us" vs "them". I definitely think it's a spectrum. I think virtually all parents practice GD sometimes and parents who consider themselves GD sometimes fall short of their goals. One of the nicest effects of GD, IMO, is that with practice you tend to extend that sort of understanding to others...including one's self.