London St Pancras - Gatwick Airport timetables haven’t been published yet but don’t worry there will be plenty of trains : 4 services an hour leaving at 00, 20, 30 and 50. All reservation-free so simply take the first one available after the Eurostar arrival (log the one you’re actually taking right before boarding).

Follow the signs for Platforms A and B : southbound trains incl. those to Gatwick leave from Platform A.

Technical note : Rail Planner tends to be (annoyingly) picky about station names. Add trains separately, first Paris - London St. Pancras Intl and then London St. Pancras - Gatwick Airport.

Here is a general guide in case : https://www.seat61.com/stations/london-st-pancras.htm :)

Thanks so much, it has been a bit confusing so this is great information.  We seem to see the same thing for Zurich to Paris which  only show very limited times and no high speed so perhaps we need to wait for better options?  

Indeed. Direct trains will be published on 26th February, domestic French trains on 7th March. See : https://www.sncf-connect.com/aide/l-ouverture-des-ventes

Direct trains have a 29€ reservation fee, domestic trains 10€ limited fare and then 20€. The latter option takes about 30 min longer (with a change in Basel and Mulhouse or Strasbourg).