Yep, we're sharing outside your domain, but that's a feature that they advertise and recommend. I've started to check GAM, but what should have been a chore in our sprint has quickly turned into a fullblown story. I'll have to find some time to waste figuring out how to do this thing that I can't believe Google doesn't have built in.
When I search to:[email protected] it shows all the documents shared with that person, then I can Ctrl+a and click "share" and remove that person from all of them at once.