Not on that list, an actual open-source alternative to Confluence and JIRA, and all of its features/plugins :(

i.e. Agile mode board, sprints, multi-project/issue keys, timesheets/time tracking, source control integration, epics and release management

Yes, some exist, but our experience is that practically none are as easy to use or feature complete as Confluence and JIRA.

(i.e. Not in the way that GitLab could be classed as a feature complete alternative to GitHub)