M2 recently stopped downloading emails OS X

Six days ago, Opera stopped downloading email from my GMail account (my
only M2 account).

I know I should probably view GMail as the culprit here, but Mail seems
to be working fine with my second GMail account that I set up as a
mirror of the Opera account (I know, not exactly a convincing
argument).

When the account is checked by Opera, there are no errors, just a
message that no messages were received.  I've gotten plenty of emails
this past week.

So, who to blame?  Opera or GMail?