May be useful for someone referencing this post.
Looks like this error is because of missing below configuration.
Apache cannot connect to a UNIX socket but instead needs to connect to a TCP Port. To allow gitlab-workhorse to listen on TCP (by default port 8181) edit /etc/gitlab/gitlab.rb:
gitlab_workhorse[‘listen_network’] = “tcp”
gitlab_workhorse[‘listen_addr’] = “127.0.0.1:8181”
Run sudo gitlab-ctl reconfigure for the change to take effect.
gitlab-workhorse is listening on port 8181. To allow gitlab-workhorse to
listen on port 8181, edit or create /etc/default/gitlab and change or add the following:
gitlab_workhorse_options=“-listenUmask 0 -listenNetwork tcp -listenAddr 127.0.0.1:8181 -authBackend http://127.0.0.1:8080”