One of our customers is experiencing a 504 error, where is that to occur?

One of our customers is having some problems accessing our web site. They've got 2 machines, and each of them is experiencing different symptoms, but I suspect that even though they're experiencing different symptoms, somehow they might be related to the same cause. I'll discuss the one where I got the most information. On that PC I installed Fiddler 2, and watched the HTTP traffic. I found an add-on that was interfering (something called "Solid Savings") which I disabled. That got me further, but not much. Next I got an error that just stopped me cold; it was an HTTP 504 error. I looked that error up and found that it stands for "Gateway time out". That's nice, but what I don't know is which gateway? Is it our customer's gateway, or is it our gateway?