Galleries North is 05:00, Galleries South & Galleries T5B is 05:30. IIRC all lounges close at 22:30.

Sometimes the North lounge is open before 05:00, as I arrived at 04:45 once before a 07:05 flight, and the lounge had just opened. They tend to be more on time opening the South lounges, with a queue forming at the top of the escalators as all the Golds wait to get their hands on the hot breakfast in the Flounge...