The problem is that all the accounts have to reside in the same Google cloud/workspace, correct? We’re currently working on using an alias as a workaround, and we may submit a feature request to Zammad to allow support for Gmail accounts from different workspaces/apps.
Also, it seems that the documentation is still missing a step, as discussed in this post: Zammad Community Post.
Thanks, and wishing you the best on your new adventures as I’m just noticing you’re no longer with Zammad 