There is an ongoing effort to get us migrated to latest stable first. So we just upgrade from 5.0.x to 5.1.x https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27480 there are still lots of warnings and deprecations to fix. Rails 5.2 will come next and I believe when we get to the point to migrate to Rails 6 it will be stable already.
There is no plan to ship with unstable/master that I know of.
Re:Gitlab 11.10 Released
by smcg-gitlab on Friday April 26, 2019 @10:44AM
(#19758045)
https://blog-rails5-upgrade.about-src.gitlab.com/2019/04/10/... is a WIP blog post about how we upgraded to Rails 5 which might be interesting, too.
I believe Gitlab is still on Rails 5.
[1] https://weblog.rubyonrails.org/2019/4/24/Rails-6-0-rc1-relea...