All below instructions are for Gitlab version 7.0.0.
As suggested on https://github.com/gitlabhq/gitlabhq/issues/6581 you can configure port on below file.
After applying all above changes still my nginx was running on port 80 only and not sure why also reconfiguring gitlab reset may all change on gitlab.yml files. Finally, file "/etc/gitlab/gitlab.rb" make this work for me.
Open "/etc/gitlab/gitlab.rb" to text editor where currently I have external_url 'http://myipaddress/' as text. I just change to
external_url 'http://gitlab.com.local:81/'
then reconfigure using command "sudo gitlab-ctl reconfigure" and voila, Gitlab is now working on port 81.
Hope this help.