OK, I'll put in my 2 cents.....by using a hypothetical situation and a few various reactions to it with various parenting styles.....obviously there are limitless variations and in betweens, I'm just trying to point out some of the more common.
Joe and Bill are at the playground. Both are 3 years old. Joe whacks Bill on the head when Bill takes a nearby toy.
NO DISCIPLINE#1: Joe's parent is completely oblivious, or sees it happen but shrugs and says "boys will be boys" and says nothing to Joe.
NO DISCIPLINE #2: Joe's parent screams from their seat on a nearby bench, "Joe, cut that out!" or "Joe, that isn't nice, stop it.", but does not get up and does not address the situation beyond that.
GENTLE DISCIPLINE: Joe's parent goes to Bill and makes sure he is OK. Joe's parent then takes Joe aside and tells him that hitting people is not OK, and that it hurts them, and briefly tells Joe some ways he can *talk* to Bill and get his point across. Joe is asked if he can think of anything that might make Bill feel better (and maybe parent suggests that apologies or hugs sometimes make people feel better). Joe's parent asks Joe if he thinks he'll be able to go back and play with Bill without hurting him, or if he needs some time to calm down. The parent is interacting by squatting down to Joe's eye level, and talking about the situation. Some GD parents do opt for time outs for physical aggression, but they can be open ended ('when you feel able to not hit", or just as a "cool off time" to separate and short circuit the situation...but they do not yell at the child during the time, or breate or shame them.) They will probably also talk about it later that night and see if they can come up with alternatives for expressing anger or frustration with his friends.
NON-GENTLE DISCIPLINE: Joe's parent storms over, yanks Joe towards Bill, yelling at him 'what's wrong with you?' or, 'stop being bad' and forces Joe to apologize to Bill. Joe's parent may (or may not) spank Joe (how ironic), while continuing to berate him for hitting his friend. Joe's parent is standing the whole time, towering over Joe. Joe may be forced to sit in a time out for being 'bad', while being scolded more, and/or Joe may be forced to leave the playground. Joe may have some completely unrelated privilege revoked that night at home.
Clearly, the GD reaction is NOT "doing nothing"...it is just NOT yelling at or hitting your child. The GD parent would also be aware enough of their child's development to complie this situation with other recent situations, and understand maybe Joe just isn't ready to socialize in this situation, or if maybe Joe is overtired or hungry, and adjust his social interactions and outings accordingly.
I think we can all agree the no discipline scenario would be the "out of control" kid, but through no real fault of his own, if he's never taught about being gentle to others...and the non-gentle scenario just teaches the child that it's oK for the parent to be mean to them, but not OK for them to be mean to anyone else (nice double standard).
Anyway, just my thoughts, take 'em or leave 'em...
ETA: Obviously, none of these reactions will guarantee that Joe will never hit anyone ever again...but it is more likely it will stop *sooner* with the GD reaction than others, because Joe is *learning* about hitting and will understand it sooner, instead of just resorting to hitting people when he won't "get caught" (non-gentle), or not learning that he shouldn't do it (no discipline). In actuality, GD is MORE effort than no discipline AND non-gentle, because it is a continuous dialogue with your child, as opposed to a quick fix.