I did find a suggestion that if it carries on for a long time with a large group of members (and I guess gmail users is probably quite a large group) then changing the wording of the email might help, as they won't then be identical to automated emails from all the other SMF forums.
If we get to that point I will try it, although I'm not convinced it would help with 'mentions' emails because they have quite a variation in wording - username, other username, thread name and link (although automated emails with links are generally viewed with suspicion by the PTB in the ether anyway). However, the structure is the same, and that may be enough.
It is all about preventing spam, and the approach does seem to be overkill followed by refining things a bit.