Display Posts Only with Specific Tag

In your template, the add_action you have there is way too late because by the time WordPress gets to your template, the query has already been performed.

If this is for a category page, I’d do the following in your functions.php:

//Display Gaming Trailers
function display_gaming_trailers( $query ) {
    if ( $query->is_category() && $query->query_vars['category_name'] == 'video') {
        $query->set( 'tag', 'gaming-trailers' );
    }
}
add_action( 'pre_get_posts', 'display_gaming_trailers' );