Output Custom Field value to shortcode

Depending on the ngg_images shortcode, you maybe need to echo the do_shortcode function.

You can try

function lilap_gid($atts, $content = ""){
$gid = get_post_meta(get_the_ID(), 'gallery_id', TRUE);
ob_start();
echo do_shortcode( '[ngg_images tag_ids="'. $gid .'"]');
$content = ob_get_clean();
return $content;
}
add_shortcode( 'gallery_display', 'lilap_gid' );