There are a few actions left before we start the GitLab application. 

Back up your secrets file (where GitLab stores encryption keys):

sudo cp config/secrets.yml /to/somewhere/safe

Install the System V init script:

sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab

Active GitLab at boot time:

sudo update-rc.d gitlab defaults 21

Make sure log files are rotated frequently (to safe disk space):

sudo cp lib/support/logrotate/gitlab /etc/logrotate.d/gitlab

Check whether GitLab and its environment are set correctly:

$ sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production

You will get an output such as the following:

System information
System: Debian 9.8
Current User: git
Using RVM: no
Ruby Version: 2.5.5p157
Gem Version: 2.7.6.2
Bundler Version:1.17.3
Rake Version: 12.3.2
Redis Version: 3.2.6
Git Version: 2.11.0
..

Everything on the system is configured to run GitLab and to make it survive a reboot.