Add image to gallery with slug

by

https://codex.wordpress.org/Gallery_Shortcode

By default the gallery shortcode will only accept IDs or if left blank will default to current page.

You need to create a plugin and register your own shortcode or download one that already exists.

https://www.google.co.uk/search?q=gallery+load+by+url+wordpress&oq=gallery+load+by+url+wordpress&aqs=chrome..69i57.9981j0j4&sourceid=chrome&ie=UTF-8