Rename Excerpt in Custom Post Type

I’m trying to rename my Excerpt field in the editor for a custom post type “slideshow” I’ve been able to rename the field globally but I’d like it to only change for the slideshow post type. I put this in my functions.php but it doesn’t do anything.

add_filter( 'gettext', 'change_excerpt_name', 20, 3 );
function change_excerpt_name( $translated_text, $text, $domain ) {

    if( $_GET['post_type'] == 'slideshow' ) {

        switch ( $translated_text ) {

            case 'Excerpt' :

                $translated_text = 'Link';
                break;

            case 'Excerpts are optional hand-crafted summaries of your content that can be used in your theme. <a href="http://codex.wordpress.org/Excerpt" target="_blank">Learn more about manual excerpts.</a>' :

                $translated_text = 'This is the link to the portfolio so that when a slide is clicked it takes them to the correct portfolio page. It should look something like /portfolio/post-name/';
                break;

        }

    }

    return $translated_text;
}