Something

SELECT t1.game, t1.text, t1.timestamp, t1.user, t2.firstname, (
  SELECT AVG(t3.rating_gr)
  FROM goozex_ratings AS t3
  WHERE t3.game =11710
  GROUP BY t3.game
) 
AS average_gr, t4.platform, t4.iconurl, t4.platform
FROM `goozex_reviews` AS t1
LEFT JOIN `goozex_userlookup` AS t2 ON t1.user = t2.fbid
LEFT JOIN `goozex_gameinfocache` AS t4 ON t1.game = t4.gid
WHERE t1.game =11710
ORDER BY t1.timestamp DESC