So apparently some (or all?) automated emails by SO are being filtered as spam by Google.
Has the team been looking into this issue?
Google gives a list of reasons this message might be spam. The most likely seem to be:
My money is on other Gmail users reporting that particular message type as spammy.
If you want to avoid having messages from Stack Exchange marked as spam, Google has some suggestions.
They also have advice on increasing the odds of emails to Gmail users getting through.