I assume BA are giving up the ground floor at Gatwick. The Gatwick lounges were put in place when BA was developing LGW as a serious second hub alongside Heathrow. With the demise of that strategy, and much more limited services from LGW, the BA lounges have been (in my experience) rather underpopulated - indeed, often rather empty. Can't blame BA for this move - hope the new First lounge is nice!