[massaman_yams]

Delivering to gmail is unpredictable when you try to send from IP space with unmanaged or poorly-managed IP range reputation, like most low-cost VPSes or residential ISP IP space.

Incidentally, I'm pretty confident the same filtering engine runs both gmail and google workspaces spam filters. Perhaps with some minor differences.

If you're paying for an email service from a respected provider like Fastmail, they almost always have professionals managing IP range reputation, so gmail delivery ends up being significantly more predictable. Make sure you set up DKIM/SPF if you're sending from your own domain.

The reasons Gmail's filters tend to generate complaints that surface on HN is:

  1) IP range reputation can frequently be a stronger spam signal than individual IP reputation  
  2) Most people aren't aware how much of an impact IP range reputation can have on delivery  
  3) There's no way to publicly check IP range reputation with gmail

Some commenters here also significantly underestimate the challenges and complexity of running spam filters at scale.

Disclaimer: I don't work for google or fastmail.