I've came across an old instance of Gitlab running on CentOS 6.
The old instance version is 8.10.5 installed using omnibus package. The omnibus package contains Postgres database, which needs to be updated from 9.2.x used in 8.10 release to 9.6.x used in the current Gitlab 10.4.4.
The documentation is not entirely complete, as I be not many people are running Gitlab instance from 2015 that hasn't been updated. There are few mentions of gitlab-ctl pg-upgrade command but the command is not available in Gitlab 8.10.5, trying to run that will provoke gitlab-ctl to give the following response -- "i don't know that command"
What I eventually figured out, and what needs to be done is to update to version 9.5.10 first -- that would update included postgres to 9.6.x
# yum install gitlab-ce-9.5.10-ce.0.el6.x86_64.rpm
Then update gitlab to the latest version.
Older version of Gitlab ship with unsigned packages and yum install command would refuse to run returning this error message -- "Package is not signed".
A workaround is to set gpgcheck=0 in /etc/yum.repos.d/gitlab_gitlab-ce.repo