Hello Jean-Baptiste,
I'm part of Marketo's Privacy Team; we handle deliverability and compliance. As Adam said, it checks the list-unsubscribe header. That said, we have done some testing with it and found that it also requires both SPF and DKIM, as well as a generally good email reputation. Marketo does use the list-unsubscribe header. Most of our customers have a good reputation with Gmail, though the specifics are individual to how your messages are performing with Gmail users.
As far as SPF and DKIM, these are settings we request our customers set up, but not everyone does so. If you're not sure if you have set these up, you can contact Support to ask them to help you check.
I hope that helps!