I guess you can only report spam through the gmail web interface which the FSF aren't using (because they're not using gmail, for obvious ideological reasons).
https://en.wikipedia.org/wiki/Abuse_Reporting_Format
How to bulk do this is interesting too. https://en.wikipedia.org/wiki/Feedback_loop_(email) says that gmail has a bulk format and that sendgrid is seeing some success.
Not defending just trying to see what a technical solution looks like
Shows you how to use googles thing if you are a sender to know if @gmail folks are reporting you. It doesnt address what to do if someone's @gmail is doing this to you (a workspace custom domain yes)... @gmail are rate-limited to a few 1000s per day per gmail address but this is still a lot obviously