I now have 3 public repos on Gitlab.com mirrored from their also-public equivalents on Github. The first two are mirrored OK (pull mirroring with webhooks that trigger upon any commit on any branch). The most-recently configured one is NOT working and there appears to be NO way to get it to work without filing a ticket.
The behavior is such that I have to do an “add origin” and push to BOTH repos directly from my Macbook (and from any collaborators); a recipe for future disaster. (Manually-triggered pull-mirroring from Gitlab UI doesn’t seem to pick up new branches, and it breaks on any force-push.)
To fix this, I would need to find the webhook URL and password on the Settings / Mirror page of Gitlab.com. The webhook’s password is not available there so I cannot configure it on the Github side.