git global setup

git config --global user.name "Konstantin"
git config --global user.email "kb@chainercorp.com"

create new repository

git clone git@gitlab.rebrainme.com:kb_at_chainercorp_com/test.git
cd test
touch README.md
git add README.md
git commit -m "add README"
git push -u origin master

push existing folder

cd existing_folder
git init
git remote add origin git@gitlab.rebrainme.com:kb_at_chainercorp_com/test.git
git add .
git commit -m "Initial commit"
git push -u origin master

push existing repository

cd existing_repo
git remote rename origin old-origin
git remote add origin git@gitlab.rebrainme.com:kb_at_chainercorp_com/test.git
git push -u origin --all
git push -u origin --tags

git log

git log --oneline --decorate --graph --all

After adding the new SSH Key in GitLab, check if you have "git" group included in SSHD AllowGroups (for Debian /etc/ssh/sshd_config). If not, add it and restart sshd (systemctl restart ssh).
Test it with

ssh -vT git@192.168.8.2

as suggested above.