Greg - SiteOrigin

The Theme Checklist plugin is an unofficial tool to help developers get their theme ready for the WordPress.org theme directory. Install the plugin, then navigate to Tools > Theme Checklist to start checking.
We’ve compiled a some of the most common theme issues that we see in the theme review queue and put them on our site, <a href="http://themechecklist.org" rel="nofollow ugc">Theme Checklist</a>. This plugin simply loads in all the latest checks and gives you an easy interface to check off the issues as you check them in your own theme. Each issue has a detailed description with examples to help you avoid the them in your own theme.
Checks are dynamically loaded from the <a href="http://themechecklist.org" rel="nofollow ugc">Theme Checklist</a> website to ensure you’re always working with the latest version of the checks. If this fails, the plugin will fall back to the bundled checks in checks.json.

Theme Checklist

The Theme Checklist plugin is an unofficial tool to help developers get their theme ready for the WordPress.org theme directory.

Eduardo Zulian

The Post Format Gallery Widget plugin creates the possibility for you to display images from your galleries inside posts saved under the format ‘Gallery’. You can select the number of images to display, the size of these images, shuffle their orders, the target of links (image file, permalink to attachment, permalink to post or maybe no linking), show captions and use one copy of the basic WordPress style for galleries, which will allow you to set up easily a number of columns.
Important: your theme must support the <a href="https://codex.wordpress.org/Post_Formats#Adding_Theme_Support" rel="nofollow ugc">Gallery post format</a>, otherwise it won’t work.

Post Format Gallery Widget

Display in a widget images from your galleries saved under the post format Gallery.

Theme Checklist vs Post Format Gallery Widget