You can change the name of add_gpp_gallery
function both in the callback and in the declaration to avoid the conflict between the original and your clone.
Something like this…
add_action('wp_head','jason_add_gpp_gallery');
function jason_add_gpp_gallery() {
if ( is_single() && 'your_post_type' == get_post_type() ) ){
remove_shortcode('gallery', 'gallery_shortcode');
add_shortcode('gallery', 'gpp_gallery_shortcode');
}
}
… should work for you.
Bonus: You can remove the original plugin action with remove_action() if needed.