Using Redmine with Gitlab

I have installed GitLab and Redmine on the same machine, using different Apache configurations on the same machine to reach by using different FQDNs. ( I can reach both of them by using these FQDNs and can login )

But I haven't found how we can integrate both for project management. I mean, at the starting phase, where the user should create his/her project. How he/she can check that the other system synchronises itself, too. ( Is there any tutorial or source that I can learn every step for using them together? )

My configuration is like:

System information
System:         Debian 8.2
Current User:   git
Using RVM:      no
Ruby Version:   2.1.5p273
Gem Version:    2.2.2
Bundler Version:1.10.6
Rake Version:   10.4.2
Sidekiq Version:3.3.0

GitLab information
Version:        8.0.4
Revision:       1ff385d
Directory:      /home/git/gitlab
DB Adapter:     mysql2
URL:            https://gitlab
HTTP Clone URL: https://gitlab/some-group/some-project.git
SSH Clone URL:  git@gitlab:some-group/some-project.git
Using LDAP:     yes
Using Omniauth: no

GitLab Shell
Version:        2.6.5
Repositories:   /home/git/repositories/
Hooks:          /home/git/gitlab-shell/hooks/
Git:            /usr/bin/git

I am looking for open-source solutions which will mostly stay my network ( the machine I have been installing Redmine and Gitlab is my focus point ), the institution I am working in, is not willing to pay money for services like zapier. Please consider the management part between Redmine and GitLab.