Good question, Matteo!
I think there were many people in open source development who were uneasy about GitHub hosting much of the world's open source code, without it being open source itself. Microsoft's acquisition brought that into sharp focus.
Moodle HQ in fact uses already uses GitLab for development and mirrors this to GitHub for code contribution. MoodleNet will be working a little more publicly, hence our GitLab being available via the gitlab.com site.
In terms of functionality and features, GitLab has a really nice way of dealing with issues across multiple projects (repositories). We're aware, though, that not everyone feels comfortable in that sort of environment, hence Changemap