I don't know what Google's algorithms are, but I'm guessing GP reached a threshold on the amount of mail being sent out within a certain time frame. I'd imagine if a person is subscribed to multiple forums and they all fired off messages with the same subject line "New Posts," that might automatically send the messages to spam without the user marking them.
For what it's worth, I've never had any GP mail sent to spam. However, I only subscribed to one forum, and I've only tagged new D&D 5e games to send me notifications, so the amount of mail being sent to me was minimal and certainly not enough to trigger any red flags.
I don't know if this is possible in the underlying code or database, but perhaps we could compile the notifications and send them out in timed intervals? For example, instead of receiving 10 emails from 10 different games each notifying me that there's a new post, I could receive 1 email saying there were 10 new posts. These notifications could be sent out every hour, or every two hours, whatever frequency is deemed appropriate. We could group these messages by time (10 posts across all games within the past hour) or by game itself (10 posts from subscribed game).
If grouped by time, the subject line could use the format "Number new posts from your subscribed games" to add some variation. Grouping by game might use the subject "Number new posts from Game System Game Name."
Personally, I prefer grouping messages by total posts across all subscribed games. That way, if they're only sent every hour, that's at most 24 emails delivered to my inbox.
