Personally, I don't buy the BA only put Heathrow hand me downs at Gatwick. The existing 777s weren't Heathrow hand me downs, they've been at Gatwick since they were delivered.

My personal feeling is they'll either order 787-10 and move the routes that are non viable at Gatwick due to the range to Heathrow (difficult due to slots), like Las Vegas (can just about be done with a 787-10 on a good day), Cape Town, Mauritius and any others which are too long or they'll go for the A350-900 which from what I can tell is pretty much a like for like replacement for the 777-200. But it's expensive to train on a new type. I am a bit of a fan of the A350 so personally hope they go for that, but suspect they won't.

Or maybe your idea of leased used 777-200s.

Personally I don't see them using 787-9s, I think they need the capacity to keep Gatwick viable.