Zero downtime upgrade/migration path from version 9.0 to current?

I'm just looking for advice here. We're running 9.0 and they've now decided they (they being a few people above my paygrade LOL) wish to upgrade/migrate to 12.x - whatever's the latest. Apparently something broke in the last upgrade and some JS automated things related to labels stopped functioning and has to be done manually. I'm still trying to come up to speed on all that.

From the reams of documentation I've gone through, it seems I cannot simply migrate the existing repositories into a new Gitlab instance. Further, it looks like it's going to be a multi-step process. What are your suggestions as to the procedure for this project?

Kind regards