functions.php conditional tag only for custom post type

I’m using gpp slideshow, which overwrites wordpress’s default gallery display. I only want to use it on my custom post type, “listings” – how would I reference it to only replace wordpress’s default gallery on the “listings’ page?

I have this code in my functions.php, and I can’t seem to get the reference right for the custom post type:

add_action('wp_head','add_gpp_gallery');
function add_gpp_gallery() {
    if( ( is_single() || is_page() ) && ( !is_page_template('page-blog.php') ) ){
        remove_shortcode('gallery', 'gallery_shortcode');
        add_shortcode('gallery', 'gpp_gallery_shortcode');
    }
}

above is the code the plugin uses, and I added it to my functions.php and tried this change:

if (is_single() && is_post_type('post_type'){

and that returns an error.

thoughts & thanks

Try using get_post_type() instead:

if ( is_single() && 'post_type' == get_post_type() ) {
    // Do something
}

The is_post_type() conditional is deprecated. But even when it existed, it returned true if the current post is any registered custom post type. It has been replaced with post_type_exists().

(More information on Post Type conditional tags.)