Choose template depending on URL

by

One post cannot (and should not) have two single post URLs. All you can change is the archive view, the list of posts.

If /gallery/ and /extra/ are post of the post type page, use a custom page template for those.

Update

Create two endpoints gallery and extra for the root (EP_ROOT) (another example). The callback function for those endpoints the has two options:

  1. If the endpoint variable value is TRUE (/gallery/ was called without a post name), render an archive listing.
  2. If there is a string value, use get_page_by_path() and fetch the post object. Render it.

Example for get_page_by_path():

$post = get_page_by_path( 
    get_query_var( 'gallery' ), 
    OBJECT, 
    'post' 
);