Clapham Junction to Brighton direct is a notable omission, I'm surprised that wasn't seen as a big enough market to retain a through service. Formerly, there appear to have been regular semi-fast (slowest, around the early 80s, appeared to be East Croydon, Redhill, Gatwick then all stations) Clapham to Brighton services since at least 1962, and probably longer.
Stopping the Gatwick Express-branded services at Clapham would be a solution, if paths permit (what's the next departure from Victoria behind the xx00 and xx30?)
Are fares on the Gatwick Express-branded services higher from Brighton to London (or, Brighton to points north of Gatwick with a change) than the former Southern services? If not, then presumably these can be treated as essentially a Southern service.