Another cool thing for me was the fact that you can use GitLab to contribute to GitLab [2].
It would be best to pick the topic you are most comfortable with (e.g. frontend HTML/JS) and after you dive in you'll definitely discover many other interesting areas you want to learn about.
You should definitely check out https://about.gitlab.com/community/contribute/ for more info on how to get started with contributing to open source. Feel free to join the Gitter room [3] and leave your questions in the chat.
Small note: My contributions led me to a job at GitLab, all the effort was seen and acknowledged and that made me feel really amazing! :-)
[1] - https://about.gitlab.com/what-is-gitlab/
[2] - https://about.gitlab.com/2015/07/07/how-we-use-gitlab-to-bui...