there is 3 ways to do it.

1. create a templage for the galleries and add the code “single-ims_gallery.php or content-ims_gallery.php”

2. use a hook to add the content at the bottom of the gallery:

 function ims_after_page_function(){
} add_filter('ims_after_page','ims_after_page_function')

3. enable the editor on the content type and add the code to each gallery

function ims_gallery_post_type_function( $post_type ){
$post_type['supports'][] = 'editor';
return $post_type;
}add_filter('ims_gallery_post_type', "ims_gallery_post_type_function");