Does your theme have a portfolio? Do you use WooCommerce products?
Custom post types
The most-used custom post types are probably WooCommerce products and various theme portfolios. They are similar to normal blog posts. They usually have a featured image and that’s all it takes for WordPress Gallery Extra to load the custom post types. Even if you don’t have an image on all of them you can still use a placeholder image in the meantime. Selling with the WooCommerce platform also adds custom taxonomies to your installation, like product tags that you can use for filtering. Furthermore, you have numerous custom fields too, such as product price. WordPress Gallery Extra can show any of these in the gallery. The WooCommerce price data is a dedicated special option that gets displayed according to your price format.
WooCommerce product gallery using WordPress Gallery Extra
This is how these products look in the back-end of WooCommerce. Your other custom post types like theme portfolios are very similar.