Nobody is claiming that gently parented kids, or indeed any kids, or indeed any humans - are perfect - though the best behaved kids I know are the ones who are gently parented well - and by that I mean have parents who hold boundaries but do it respectfully. That is basically what it comes down to.
What you are talking about there is authoritative parenting, which is consistent, attentive, and not harsh.
Parents who cling doggedly to the label 'gentle parenting' are doing so for reasons within themselves, maybe trying to differentiate themselves from their own parents, or the parenting of the culture they were exposed to as children. Or maybe they confuse the terms authoritarian and authoritative.
Gentle parenting springs from positive psychology. The parenting form, positive parenting, advocates ignoring the bad out of faith that despite all indications to the contrary, the child will one day turn his behaviour around himself, thanks to modeling of dealing with emotions and interpersonal interactions, hugs and assurances of unconditional love while the child is emotionally disregulated, and comments on the child's emotional state instead of reprimands or firm requests to stop unacceptable behaviour or speech, and avoidance of introducing any sense of shame into the child's life, which is what is said to happen when you issue a rebuke, a firm, "Stop that!" or any comment directing the child to take note of the effect their behaviour has had on someone else. Any intervention prompted by behaviour is supposed to make the child aware of his emotions and ultimately to learn to regulate them himself. Shame is old school, and bad.
(I'm not talking about screeching or sarcasm or insults leveled at a child here. I'm not talking about humiliation - I'm talking about any invitation to a child to 'Treat others as you would like to be treated,' however it's expressed - 'Look at Daisy, she's crying because you bit her and her arm hurts.')
What you get after 8 years or so of that is kids who really haven't learned that other people are real, three dimensional beings who are emotionally and physically hurt by being bitten, and are really surprised - actually outraged - when they get bitten back.