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).
I did some tiny digging because I remembered that there is a way to report individual messages in a structured machine readable way to abuse@ for these things --- i suspect that this is technically supported by gmail (if not given a lot of signal weight)
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
