DoodleDogCody on "Conditionally adding shortcodes"

I use a plugin on a lot of my sites that modifies the normal [gallery] shortcode.

On a current site, I would still like this to occur on every page on the site except for a specific post type.

I was thinking that I could do something like this.

if( !is_singular('custom-post-type') ) {
   add_shortcode('gallery', 'shortcode_function');
}

But this does not stop the new gallery shortcode function from firing. Is this possible to do at all? If so, can someone help point my in the right direction?