It's bad enough that Google Drive/Docs notifications are treated as "external senders" so you have to open up your internal Google Groups to the world; then Google Groups flags them as spam anyway!
I'm guessing spammers may use Google Docs comments as a platform, but have never seen actual unsolicited Docs comments.
In my experience, it is more they "share" spam documents with you.
Nobody can send emails to Gmail with any certainty. Not even Google.