The Sussex Route Study (
link) concludes that substantially more services are required to run off-peak through Gatwick from London towards to the coast by 2043. Figure 43 of the document suggests that this could be a total off-peak service of 24tph formed of:
- 2tph Thameslink to Horsham
- 2tph London Victoria to Bognor Regis and Portsmouth/Southampton (splitting at Horsham)
- 1tph London Bridge to Bognor Regis and Chichester via Horsham (splitting at Barnham)
- 3tph London Victoria to Haywards Heath
- 1tph London Victoria to Hove
- 1tph London Bridge to Hove
- 2tph London Victoria Littlehampton via Hove
- 1tph London Bridge to Littlehampton via Hove
- 4tph London Victoria to Brighton
- 4tph Thameslink to Brighton
- 1tph London Victoria to Seaford and Eastbourne (splitting at Lewes)
- 1tph London Victoria to Hastings
- 1tph London Bridge to Eastbourne
Amongst this there seems to be no point in running an express shuttle between London Victoria and Gatwick unless a fair number of services towards the coast run non-stop through Gatwick.
The service level shown would require upgrades to the Brighton Mainline, but I think that those upgrades would be required anyway to meet peak capacity requirements.