Can you point me at any data which supports this? It seems like it should be true, but GHC didn't appear to see a year-over-year increase to contributors when they switched from Phabricator to GitLab in December 2018.
Another thing that we hear from people considering a move to GitLab is that they really like how fast GitLab moves (one release per month!), and how it works with the community to keep improving the product. I recently joined as the Sr. Open Source Program Manager as part of the Community Relations team, and we're thinking a lot about how to continue to improve the experience for our community.