Hi Howski,
The Gatwick Express train from Gatwick ends up in London Victoria Station. It takes around 30 minutes to get to London.
Transair happen to have an excellent well stocked pilot shop located about 5 minutes from the train station.
From memory as you leave the platform that the train arrives on, turn left and head for the escalator and stairs that take you to the food / shopping area of Victoria Station (upstairs). Go up to the first floor of this area and walk all the way to the other end. You will end up walking out of the station onto a busy road. Turn left and walk over the road. Then a right and a left again. About 2 to 3 minutes walk down that road you'll see Transair pilot shop on one of the corners (can't remember which corner though!).
Buy your stuff in the pilot shop and the walk back to the station will seem like an endless journey weighed down with tons of equipment!!!
Best wishes,
Charlie Zulu.