Image descriptions are stored as content (field post_content) while captions are stored as excerpt (field post_excerpt). This means that descriptions are shown on single attachment pages, while captions are shown in galleries and if image is embedded in a post content with a [caption] shortcode.

If you use images just in galleries, in order to have text below image in both places, you need to fill both fields.

This plugin changes that. You only fill description field and that text is used as caption of image in galleries.

There are no options or settings, just enable plugin and everything works automatically.