Amazing what a Google search can provide ...
It's at the Sofitel Hotel, which means exiting the terminal and going through the car park (not as dismal as it sounds).
http://www.businesstraveller.com/new...l-arrivals-lou
Who can use it?
BA and Emirates business and first class passengers, and Delta Business Elite customers. There is a possibility that other airlines may be added to the list in the future, including Continental, which has recently moved all its Gatwick departures to the North terminal, although at present its arrivals continue to come into the South terminal (where Continental has its own lounge).
It's also available to Priority Pass cardholders.
About the sniffy comments on lounge dragons, the arrivals lounges I have used have been landside. Arrivals lounges deal with arriving passengers. There is a clue in the name. They do not offer the same airline services as at the airport.