What time is your Eurostar reservation? Also, I am unaware of any Eurostar leaving Victoria. I beleive you'll need to get to Waterloo station either by tube or by taxi to meet the Eurostar. The trip from Gatwick to Victoria is about 30 minutes and another 10 on the train Victoria to Waterloo. I'd make that 30-40 minutes including time to get down to the tube, wait for the next train, get out of Waterloo tube and to the Eurostar station. If you are taking the 15:11 out of London you should be okay, but that is cutting it close. I would expect you coulld get to the express from Gatwick earlier than 1:40 (13:40). There are 4 trains leaving between 1 and 1:20 which hopefully you can get. I usually get out in about 1 hour to 1.5. Good luck.