add_action('save_post', 'createGallery');

function createGallery () {
    global $post;
    if ( $post->post_type == 'activity' ) {
        $gallerypost = array(
            'post_content' => 'the text of the post',
            'post_status' => 'publish',
            'post_title' => 'Photo album',
            'post_type' => 'post',
            'post_author' => 1);
        wp_insert_post( $gallerypost );
    }
}

do_action('save_post', $post_ID, $post);

add_action('save_post', 'createGallery',1,1);

function createGallery ($post_ID) {
    if ( get_post_type($post_ID) == 'activity' ) {
        $gallerypost = array(
            'post_content' => 'the text of the post',
            'post_status' => 'publish',
            'post_title' => 'Photo album',
            'post_type' => 'post',
            'post_author' => 1);
        wp_insert_post( $gallerypost );
    }
}