I am flying EZJet from Gatwick to Pisa in June - and am wondering if I should buy the train ticket from Victoria to Gatwick before I leave the USA? Or at a travel agent in London?

Thank you!

No need to buy the ticket in advance. Buy it before boarding the train at the station.

easyJet offers a 10 per cent discount (it says) for the express train booked online at http://www.easyjet.com/EN/Planning/T...atwickexp.html
You can print out the ticket, saving some time at the station.

10% off a Gatwick Express ticket is still more than the full fare on Southern

Yep as flanneruk points out the local train is cheaper. You're after the London Victoria to Brighton train, it calls at Gatwick airport, and takes 30-35 minutes. A single off peak fare costs £13.20, whereas the Gatwick express is £16.85 after the discount. I don't think there is even a difference in journey times.