Marketing Cloud Sends delivered to Gmail Spam folder
|Knowledge Article Number||000229842|
This is related to content and not reputation. Gmail uses a combination of subscriber engagement, the address book, and content heuristics to determine if an email is Spam. There's no "unblock" type approach that can be used as with other domains.
Contacting Gmail: Gmail has a Bulk Sender Contact Form, and on this form you can insert the effected domains, and 2 .txt files of sample emails. Generally, the response will state that they rely on Users to identify Spam for them and that if enough people have marked your mail as such, they can't fix this.