It's taken a month of trial and error but I finally got all our repos migrated to Gitlab!
Mind you, the code in question is 10-12 years old and I wanted to get all of that history including the branches migrated, as all of the original authors have since left the company. The commit history is all we have to understand the code base.
I found a working albeit flaky utility to help with the extraction of the source and history, but as often happens, at least to me, none of the documented use cases fit my situation, so I tried several different variations of parameters until 1 worked.
If I had a win this week it was this. It's the first, and in some ways the most important step in a much larger initiative. More on that as it progresses.