See how to add images to the gallery:
How to add a gallery of images to your listings
The option in your screenshot is shown here:
How to automatically output listing images in a gallery
When you view your listing the gallery is output onto the template using through the following gallery hook:
<?php do_action( 'epl_property_gallery' ); ?>
Resulting in a gallery of images appearing under the Property Features on the default single listing template. E.g:
https://easypropertylistings.com.au/demo/property/9-ashmore-way-sorrento-wa-6020/
The gallery function is basically using the WordPress [gallery] shortcode.
Does the gallery appear on your listing if you add the [gallery] shortcode to the text area?