Let's say you need to add a paragraph or two and the gallery.
The first thing that comes to mind - do the gallery and under the second paragraph of custom fields(they are the same custom post fields) and apply to them what do you need formatting in the template. Themselves wrapped in custom field metaboxes with textarea and You will be happy.
The option is a bit more difficult to use Post Formats
. For example, the text from the body of the post is cut by the delimiter is a newline(mean expload('\n', $post) ) and then both parts are derived anywhere.
I personally think the first is easier. If you go this route, to help you create metabox-ov can try to use a metabox.io