There are some high-profile FOSS projects using GitLab, but they're not as visible as they would be on GitHub because they host their own instances:
- https://gitlab.freedesktop.org/
- https://salsa.debian.org/ (AFAIK this is an ongoing migration)
Others are considering a migration:
- KDE: https://gitlab.com/gitlab-org/gitlab/issues/24900
- GNU Emacs: https://gitlab.com/gitlab-org/gitlab/issues/28152
Maybe someday we'll have federation to work around this: https://gitlab.com/gitlab-org/gitlab/issues/30672 :-)