How do I use Gitlab server as a central staging server?
Usually, people set up a remote named "upstream" for the source, in this case it would be https://github.com/drupal-composer/drupal-project.git. Remote "origin" should be the git URL of your Gitlab repo. So for you, here is a sample workflow:

The setup:

git remote rm origin
git remote rm composer
git remote add composer https://github.com/drupal-composer/drupal-project.git
git remote add origin https://gitlab.example.com/user1/drupal8-test.git


The result:

$ git remote -v
composer    https://github.com/drupal-composer/drupal-project.git (fetch)
composer    https://github.com/drupal-composer/drupal-project.git (push)
origin  https://gitlab.example.com/user1/drupal8-test.git (fetch)
origin  https://gitlab.example.com/user1/drupal8-test.git (push)


Get upstream files:

git fetch composer
git merge composer/8.x


Adding your own files:

touch myfile.txt
echo "hello world" &gt; myfile.txt
git add foo.txt
git commit -m "Added my file."
git push origin master