Over the weekend, we have received reports via Google Groups that many Gmail users are having issues logging in to Gmail to check their emails.
Many of these users are receiving 502 Errors on their login attempts.
Gmail Guide has confirmed the issue:
I'm sorry to hear that some of you are unable to access your accounts due to 502 errors. Our team is currently aware of the problem and we are working to get things back up and running for you. Please visit our Gmail Updates and Alerts subgroup for up to date information on this problem.
Currently the status of the 502 errors on login at Gmail Updates and Alerts is nada. Keep watching it for updates and I hope it is resolved shortly.
Forum discussion at Google Groups.