What does gentle discipline have to do with attachment parenting?
In my heart and mind? Everything.
Attachment Parenting is about Relationships. It is about fostering a trust based relationship rather then a fear based hierarchy. It is about coming along side your child as a person worthy of respect like any other human being.
Punishments bring results, but at the cost of a truly trusting relationship. Punishment may cause a child not to do that again (only if they are developmentally able to do what you expect of them) but it won't give them the true reason that particular action isn't acceptable, only an arbitrary one.
I don't force sharing. DS1(5.5yo) has toys that belong to him, he chooses if some one can play with them and for how long, but if he is a dictator then his brother often doesn't want to play with him.
DS2 (1.11yo) has the same option but often shares, though only for short stretches of time.
We also have Family Toys, that must be shared. But it is a first come first serve thing. And the child can play with it as long a they want but once abandoned it is free game.
Not forcing sharing has allowed the boys to begin an understanding of personal property, though they don't fully understand this yet, it is laying the ground work. Ds1 is also learning that not sharing often make people not want to play with him. When feeling good he is getting much better at sharing.
Yes this method takes longer, means a lot of time running interference. A lot of time teaching and reminding. But they are learning the real life consequences of not sharing, and not respecting personal property, waiting for your turn. Rather then learning to share or not get to play, only to wait till later in child hood that not sharing where mommy can't see means you loose a good friend rather then simply make your brother mad for a few minutes.
Setting arbitrary punishments rather then guiding them through natural consequence means that they will have to learn these lessons the hard way in the real world with more painful and long term effects.
Helping them to see and discover the real world consequence (within age appropriate reason) saves time and hurt later in life.