I don't work at gitlab but one great avenue is working on large open source rails apps. And gitlab is actually open source. Try to get it up and running and tackle an issue they have.

https://gitlab.com/gitlab-org/gitlab-ce/issues

also some other rails apps to look at:

discourse https://github.com/discourse/discourse

dev.to https://github.com/thepracticaldev/dev.to

just recently mentioned in another thread consul https://github.com/consul/consul

Or search for large open source rails apps.