It's been going around.....I had the problem one day and then "poof"....not had the problem since. Try rebooting and see if that helps. Seemed to work for me a couple of times.
Bunch of causes for a gateway error, from simple lag, to your browser having a hard time talking to a server, or two different servers on a website failing to communicate properly with each other, simply put.
A couple things you can try at least to see if its on your end is clear your browsers cache and clear cookies, but most of the time its not on your end.
If that does not work we can try other possible fixes.