What's the dealio on importing data from a legacy issue tracker system into Gitlab CE?
Do tools exist for this? Schemas? Suggestions?
Please notice that this is really a legacy issue tracker system. It predates bugzilla, and runs on an old IIS server and SQL Server 2000).
(Say whatever you want about this setup, but it's nothing we haven't already heard.)
You should be using the REST APIs to create your migrations.
Generally recommendation questions are off topic, so if I mention there is a redmine issue importer and there are issue tracker issues on the gitlab ce issue tracker requesting this. This sounds like a good kind of thing to make as a community contribution if it's a popular tool.
But if it's not, and you're the only person in the world using your tracker, you probably will want to study the python based redmine issue importer it may server as an example for you to write your own REST-api based tool that reads your db and creates the Gitlab Issue Tracker issues. You don't want and don't need to know the Gitlab side's PostGres schema. It will change over time anyways.