While you are using an ACF WYSIWYG you’re not using an ACF gallery field. Just a basic WP Gallery. From your description, it is working like it’s supposed to.
To show only images that you want to show, or images that are attached to other posts you’ll need to use the “include” setting that is shown on the page you linked to. To do this you can click on the gallery edit icon to select images. The “menu_order” value specifies that the order should be the order you set them to by editing this gallery as well.
