Gmail is currently down for many users and it shows a 502 error message:
"We're sorry, but your Gmail account is currently experiencing errors. You won't be able to use your account while these errors last, but don't worry, your account data and messages are safe. Our engineers are working to resolve this issue. Please try accessing your account again in a few minutes."
Some Gmail accounts can still be accessed using other interfaces: basic HTML, mobile version or IMAP, but you can't receive or send messages.
The problem seems to be widespread, since many people are reporting it on Twitter and Gmail's group. "Gmail Temporary Error (502). Last time I saw something like that from Gmail was 2004," says José. "All my Gmail accounts are down. 502 Temporary Error. for nearly an hour already," complains Jonas. "bitful is all grrr because Gmail throws a 502 (temporary error) and wonders if the cloud can be trusted after all."
Gmail Guide says that "the Gmail team is currently aware of a subset of users being affected by the 502 error on login. (...) Engineers have pinpointed the issue causing the 502 errors and it looks as if users should start to regain normal functioning of their account."
Update: After more than an hour, Gmail is back, but the outage is in Google Hot Trends.
TechCrunch has more information from Google:
"Since about 2 p.m. Pacific Time today, many Gmail users have been unable to access their email. We are very sorry for this interruption in service. The issue is being caused by a temporary outage in the contacts system used by Gmail which is preventing Gmail from loading properly. We are starting to roll out a fix now and hope to have the problem resolved as quickly as possible. Even though you may not be able to get to your inbox right now, your mail is safe, including new incoming messages."