We will be taking the Southern train from Gatwick to Clapham Junction and then transferring to an overground train to Shepherd's Bush. Do we need to tap in or out at Clapham Junction or do we just go to the platform for the overground train and continue on our journey?
You only have to tap out if you leave the station, so you'll be OK just changing platform.
When you change trains at Clapham Junction, tap your card on the pink card reader. This is to indicate that you won't be travelling via zone 1 and will save you money- more info here: https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets-and-oyster/pay-as-you-go/touch-pink-card-reader-when-changing-trains
You should see a pink card reader on the platform for the overground train.
Thanks! I checked our trip and tapping the pink reader will save us a couple of pounds each.