Fetching review value using wpdb class

You need to add the global $wpdb reference and also add the second parameter required for prepare():

global $wpdb;
$post_id = $post->ID;
$reviewScore = $wpdb->query(
    $wpdb->prepare( 
        "SELECT review FROM {$wpdb->prefix}gdsr_data_article WHERE post_id = %d",
        $post_id
    )
);