There are no plans to migrate all of GitLab to Go. The main Rails app is going to stay a Rails app for the foreseeable future. There are a few reasons for this. For one it'd be such a huge project, but also Rails is working well for us, it's great for our pace of feature development.
We are working on moving the git layer to Gitaly[0] which is written in Go (and is what this blog post is about). It was one of our major bottlenecks and we've seen a lot of benefit from having made the switch. It's not done yet, but a lot of the calls to git that the application makes are now done through Gitaly.
They wouldn't need to stop the world and do a full rewrite. It would be feasible if they stop writing new components in Ruby and began replacing the existing parts piecemeal.
We are working on moving the git layer to Gitaly[0] which is written in Go (and is what this blog post is about). It was one of our major bottlenecks and we've seen a lot of benefit from having made the switch. It's not done yet, but a lot of the calls to git that the application makes are now done through Gitaly.
[0]: https://gitlab.com/gitlab-org/gitaly