Everyone lets get a few things clear : BA Gatwick Fleet is not a franchise or subsidiary its Mainline and part of ACC NSP that was a condition in the new fleet setup !
Franchises are GB, BMED , Comair & Sunair
Subsidiary - BACONNECT
The only way a franchise will become a subsidiary is that BA buy them for example : Cityflyer was bought for £ 70 million by ba which became a subsidiary and then they merged with EOG (European Operations @ Gatwick ) another subsidiary to form Eurofleet LGW (mainline operation) and now the Gatwick Fleet - Mainline.
Re Slots at LGW BA Mainline hold more that GB which I believe are not doing well , but are making extra money as BA Connect !
Remember all European flights into both LGW & LHR still offer Club Europe / Euro traveller.