What are the tools and libraries that work for Gitlab ?
If you want to see packages to interfere with Git, see its own git page.
Get the list of open issues, reference in a commit message at "Fixes #"
See git-commit-insert-issue, which works for Github and Gitlab. It has a commit minor mode and an interactive function to fetche the opened issues of the current project when you type something like "Fixes #" in the commit message.
glab.el, a minuscule Gitlab library
See glab.el. It's for Emacs 25 and onwards though. It's neat on how it handles the authentication (quite transparently for the user).