Sorry for the delay answer,
I assume your post type gallery is using slug "gallery"
each user has a gallery post, which is already created, and author is that user.
please try this:
add_action('cred_save_data', 'my_save_data_action',10,2);
function my_save_data_action($post_id, $form_data)
{
// if a specific form
if ($form_data['id']==123)
{
$current_user = wp_get_current_user();
$current_ID = $current_user->ID;
$parameters = array('author'=> $current_ID, 'post_type' => 'gallery');
$posts = get_posts($parameters);
if($posts)
{
$gallery_ID = $posts[0]->ID;
update_post_meta($post_id, '_wpcf_belongs_gallery_id', $gallery_ID);
}
}
}
Please replace 123 with your CRED form post ID.