Omg thank you for this thread, I have exactly the same question. Gentle parenting seems great and it's very much my style, but it doesn't actually tell you any techniques for managing bad behaviour, it just seems to imply that if you're doing it right, it won't happen in the first place, which, er - what?
For example 21 month old can be quite defiant. We're not shouty or authoritarian and communicate a huge amount with him, always make sure to explain things very very well, give him lots of notice of what's happening next... but he will do things like - he knows he is not supposed to hit either of us. He knows it is wrong, we have read the 'hands are not for hitting' book 100 times, we have explained and explained it hurts, it's wrong, we don't do it, gentle hands etc. But it's not aggression as such - he will look at us, get a gleam in his eye and do it, specifically to get that reaction, to see what we'll do. And before anyone says its attention seeking, I can be sitting on the rug with him, playing blocks, absolute quality 1:1 time and he does it.
So.... how do we handle that then? Seriously, I would love a book or method recommendation please, because it's not very useful when it's just 'oh, try to understand why your child is frustrated/feeling aggressive' when that's not the issue, I 100% believe it's a boundary testing thing. Sorry for the derail op, but - help!