Some of the users are being faced with a "Temporary Error (502)" error along with a flurry of other error codes as well. However, the POP and IMAP protocols have reportedly been working fine.

Over at the Google Groups, Gmail Guide has put up several posts addressing this issue. Here is what the Gmail Guide has to say about the inconvenience caused to the users:

The Gmail team is currently aware of a subset of users being affected
by the 502 error on login. Our engineers are looking into the issue
and we will provide updates here as they become available.

Update: We are still investigating the 502 errors, but are receiving
reports that users are still able to access their accounts via IMAP,
POP access, and iGoogle.

Update - we have confirmed that affected users may also be able to
access their account with Gmail's basic HTML view. You can access this
via:

Summary - users are receiving 502 errors when attempting to access
their accounts via the web interface. There are a few ways that you
may still be able access your account:

1. Basic HTML view
2. IMAP access
3. POP access
4. iGoogle"

Update - engineers have pinpointed the issue causing the 502 errors
and it looks as if users should start to regain normal functioning of
their account. Thanks for being patient while we worked to sort out
this problem.

Users who were temporarily affected by the 502 errors should now be
able to access their account. Thanks for your patience while we worked
to resolve this issue for everyone.

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