GitLab Hackathon is a virtual event that takes place once a quarter where community members get together to work on merge requests, participate in tutorial sessions, and also to help new community members. The event is open to anyone who are interested in contributing code, documentation, transations, UX design, etc. to GitLab. The main communication channel during the Hackathon is the GitLab Community room in Gitter.
If you want to get more familiar with how you can contribute to GitLab, please visit the Contributing to GitLab page.