Monarch had 154 slot pairs (not total slots) a week at Gatwick which is equivalent to about 22 departures/arrivals a day. So room for quite a bit of expansion. On some days in Summer departures were higher than 22, more like your 25 but less of course in Winter. I think Monarch based 10 aircraft at Gatwick during the summer each operating 2 to 3 departures a day (so 2 or 3 slot pairs).
With greater flexibility of scheduling as a result of these new slots BA could put together an impressive long medium and short haul schedule. I expect planning has been going on longer than we realise. I remember when Easyjet bought the Flybe slots BA was interested but said they had come about too early for their plans. With BA Gatwick now in much better shape and I believe profitable it is now time to put plans into action. However I expect other IAG airlines may get a look in. I am not convinced about Level turning up that quickly or at least substantially. The refurbished, densified and fully paid for BA 777s will allow them to compete more effectively with Norwegian. The BA brand is strong at Gatwick and certainly long haul lucrative. I doubt if they would wish to dilute that. I also expect some Airbus aircraft will be transferred from Heathrow or leased in to expand short haul network. New A320/A321 are due at Heathrow so others can be kept longer if necessary. I am also not convinced about Vueling/Iberia Express expanding greatly. BA is popular and known at Gatwick. Again why undermine it? That is not to say that other IAG airlines will not expand their presence.