This will do the following.
1) If a post has not been edited yet it will cause the front end output of the gallery field to automatically hold all of the image attachments for the post.
2) When a post is updated in the admin it will automatically load all of the image attachments for the post and put them into the gallery field where they can be edited and saved.
Like I said in my first reply, there is not automatic way to have this happen on all existing posts. This is the same as any ACF field. If you have posts that already exist, creating a new field does not automatically set values in those already existing posts. There is nothing in ACF or WP that will do that.
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