This tweet is, unsurprisingly, too light on details. I wonder what the plus for Xcode is here? I'm sure it already had Git integration. Is Xcode getting project, issue, merge-request etc. management support? And they chose to start that with (or exclusively base it on) GitLab?
Maybe they had a slide for GitHub and they deleted it in light of the Microsoft announcement. Apple is one of very few companies secretive enough to be able to do something like that.