Someone else asked the same question, which both DZ and I answered here: hashnode.com/ama/with-gitlab-cj7uu549m01pq2qwurkk…
We're actually seeing a lot of open source projects moving to GitLab. Some times on their own instance, some times on GitLab.com. But we're still a magnitude smaller than GH when it comes to open source projects on GitLab.com. We believe it's a matter of time. GH has been around for a very long time and gets the most eyes still. And we understand that, so we've made it easy for you to mirror your repository between GH and GitLab.com.
We're making sure that GitLab.com is becoming more stable and much faster. By building the best possible place for someone to build and ship software, we believe it's a matter of time before more open source projects will move to GitLab.