<?php $gallery = get_post_meta($post->ID, 'gallery', true); ?>

<?php echo do_shortcode("[smooth=id:$gallery;]"); ?>

Solution - 1

This doesn't work because what you put in your post ( [smooth=id:yyy;] ) isn't a shortcode. It's just a filter on the_content :) Just make a custom loop using wp_query where you want to display your gallery and insert just the one page that has [smooth=id:yyy;] in it. After that exclude that page from the menu and you should be set... sort of... Or just use a different gallery plugin for nextgen: http://www.cozmoslabs.com/2010/01/13/wordpress-photo-slideshow/ It can be modified quite a lot ( it's based on http://spaceforaname.com/galleryview )

Solution - 2

I can't get this plugin working (keeps saying it only works with NextGen Gallery - which I have instaled), so I can't test the code. I looked through the code and like Cristian said it adds a function to the content filter, so I thing this should work:

<?php
$gallery = get_post_meta($post->ID, 'gallery', true); ?>

if($gallery){
	echo apply_filters( "the_content", '[smooth=id:$gallery;]' ); 
}
?>

Solution - 3

This should work (hope so). Give it a try...

<!-- Get the post meta gallery Id -->

<?php $cf_getGalleryId = get_post_meta($post->ID, 'gallery', true); ?>

<!-- Check if it relly does exist  -->

<?php if ($cf_getGalleryId != Null) : ?>

<!-- Execute the shortcode for that gallery Id  -->

<?php echo do_shortcode('[smooth=id:' . $cf_getGalleryId . ';]') ; ?>

<?php endif; ?>