Also, to remove the problem of false positive, refuse spam at the SMTP connection level and do not use a spam folder. This way users will get an MTA email in case of false positive.
On my Fastmail account, 99% of spam comes from a gmail address. I imagine there's a lot more that are filtered upstream with a similar DNS check, but my point is I receive most spam from legitimate addresses on the world's largest email service.
This provides natural pressure for gmail to reduce their outgoing spam rate. I'm sure they do try to get outgoing spam down. Spammers are just a resourceful bunch...
Any large free email provider will have these problems. It's a reason to use your own domain for email, with your own reputation, instead of sharing your reputation with the whole world, including spammers.
I am not doubting you, but did you check that the email was actually coming from google servers?
I don't do any kind of content based filtering in part because some of my users do anti-spam, anti-phishing work, so of course they need to be able to talk about spam and phishing, and they need to forward along spam / phishing, without worrying about filters.
Also, every email, accepted or not, has specific reasons for what happens to them, not a vague set of rules that nobody knows like Gmail has.