Messed-up installation on Debian 9

I just tried installing GitLab the very normal way and got into trouble: The problem is that I can’t tell what happened on my system or what is currently happening. I’m on Debian 9 (Stretch), this is my uname -a output:

Linux Me-Server 4.9.0-6-686-pae #1 SMP Debian 4.9.82-1+deb9u3 (2018-03-02) i686 GNU/Linux

I followed this official guide and added the repositories to my system. Neither gitlab-ee nor gitlab-ce could be found, so I executed (as root) apt-get install gitlab, which seemed to work fine. But! In the post-install script, which I found out to be located at /var/lib/dpkg/info/gitlab.postinst, there was an error occurring with one of the sed -e commands. After commenting out lines 176 to 185 and executing dpkg --configure gitlab again, it went on for a bit longer but eventually tripped over some problems with runuser in /usr/lib/gitlab/scripts/, which I could not fix. It seems like this script creates the database, so I don’t have a complete or usable installation at the moment.

My big problem is that apt-get remove gitlab now apparently triggers a script that endlessly spawns /bin/sh /etc/init.d/gitlab stop commands, which does not only prevent the uninstallation from completing, but also renders my system almost unusable after some time. My reaction was to reboot quickly, but now I’m having the problem of the incomplete installation and uninstallation. Am I doing something wrong or are these problems actually bugs in the dpkg scripts provided by the GitLab repository?

I would very much appreciate your help, as I will otherwise just revert my system to a recent backup and forget about GitLab, which seems like a real pity. I just don’t know anything else I could do to fix this problem…

I just found out that I was not even able to install GitLab from the repository provided in the guide! I installed a package called gitlab (not gitlab-ee or gitlab-ce), which most probably came from my Debian 9 repositories, here it is. But then: Why does apt not recognise the repository? I have opened the following file to verify:
In there, the fifth line says “Label: gitlab-ce”, so if apt parsed this, the package gitlab-ce should be available. Help…?