Hi Matthew,
When you switch from one cap (zones 1-2) to a further out one (Gatwick) then the journey that takes you there will be charged in full, until you reach the new cap.
It is just possible that in the second case (Underground and Thameslink journey) the back office calculation might recognise that the Underground part ought to be treated as part of the cap. If this happens then contactless will charge the rail only fare straight away, while Oyster will queue an adjustment to be credited back the next time you use the card, but only if it’s a normal adult card. The reason I can’t say for certain is that the extension fares charged by GTR have been fiddled to avoid an unintended consequence of the really cheap fare between East Croydon and Gatwick Airport. If you have the time then it’s always worth stepping back a train at East Croydon so you can touch out and back in again and split the journey into two journeys.