Issue
I have not found a way to disable the automatic startup and it ends up using too much RAM when I'm not using.
The init files are not inside /etc/init or init.d.
I try update-rc.d gitlab remove and no results.
I am using GitLab 8.5.4 in Debian 8.
Solution
Problem is solved!
I contacted the GitLab by their official page on Facebook and here is the answer.
I am using GitLab in a Desktop and it was using ~700MB.
If you too want turn off GitLab on startup, just execute in a terminal:
sudo systemctl disable gitlab-runsvdir.service
Answered By - Natan Felles Answer Checked By - Mary Flores (WPSolving Volunteer)