Does it say anywhere why they chose Gitlab? I used to be a big fan of them, but they kind of died for me after they implemented big limitations on the free tier. I stopped being able to Trojan-horse it to my company, and Github got good CI and other nice features, so I've switched to that.
It's not clear if Mediawiki are self-hosting gitlab. If they're relying on Gitlab (the company) then I'd be worried about Gitlab running out of money and going out of business. I had a bit of an interaction with one of their sales people a while back and they appeared desperate. If self-hosting, there would be a bit of uncertainty while development reorganizes but I don't expect any long term problem.
