This will not run in a usable way on a RaspberryPI 3. Maybe one day. Save yourself the trouble and if you
want to run your own GitLab server (basically your own GitHub), buy an older used machine with an I7 processor
and plenty of diskspace (500GB min) to hold your projects. GitLab really needs a dedicated machine. Don't try to
install other software on it.
When you checkout a GIT repository you have a complete copy of the repository. This means if your server crashes, you can restore the
repository on the server from your local copy. Still, backup the server periodically.