That's also not true. KKG isn't some super move that is guaranteed to fall an enemy. KKG by itself isn't as strong as 51 minutes of beatdown. It is a massively powerful blow, yes, but it is still one blow and can't be compared to Luffy throwing all of his arsenal at you in an almost-hour-long beatdown. If KKG was the first thing Luffy used, Doffy would not have fell unconscious. It also isn't Leo Bazooka that stunned Luffy and caused him to repair himself with strings again, it's the entire beatdown Luffy inflicted.
KKG also seems to be a one-shot maneuver, as G4 ended right after Luffy delivered it. It could be because he was so tired when he did it, or on purpose, granted. But Doffy can just fly around it and avoid getting struck by Luffy at all, especially when he's healthy. Doffy wouldn't be able to land many hits on Doffy once Doffy has knowledge of how his attacks work, how his attacks will affect him, and how long G4 will last. Luffy just isn't strong enough for Doffy yet.