This has some very detailed instructions on how to install and setup GitLab on FreeBSD but there is a port available as well (https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=202468 https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=208511). Should the above page just refer to using the port?
portsnap fetch extract (update if you've already extracted before)
cd /usr/ports/www/gitlab && make install
I'm currently in the middle of installing this via the port so I can't say if it works 100% or not but it sounds like it should be a stable solution as of earlier this year https://forums.freebsd.org/threads/50920/page-2