[stomachmonkey]

Gatwick to London is 1- 1 1/2 hours by car.

Express train to Victoria Station is 1/2 hour and they run regularly so under an hour.

Give yourself 2 hours round trip and you want to be back at Gatwick at least an hour before boarding so I'd say all in you have 4 solid hours to spend in London.

Make a list of things you are most interested in seeing then check proximity of them to each other.

Stopping in a proper pub for a pint is great, eating, meh, the Brits are not renowned for their cuisine so you won't miss much there.