In 2016 I think it would make more sense to just make Redmine + Gitlab interaction richer.
Gitlab has:
1. Merge request workflow.
2. It has built in issue tracker but you can turn that off and link it to Redmine so you don't have to migrate away from Redmine's rich features.
3. It has a build automation system that can automate build, test, and deploy on a multi-fork, multi-branch git repository ecosystem, something that would be hell on earth to build into Redmine.
The reason we still use redmine:
1. All our wikis are in it, and our existing artifacts (bugs, tasks, epics, sprints).
2. We want one redmine project with multiple git repos, whereas gitlab's built in bug tracker is one-set-of-issues-per-git-project.
Currently redmine's git integrations are useless to us. But a REST api interaction between Redmine and Gitlab are incredibly rich in potential, and anyone interested in collaborating on that should contact me via warren dot postma at gmail dot com