https://about.gitlab.com/downloads/#centos7

Step 0: (Optional)
sudo yum install wget -y

Step 1:
sudo yum install curl policycoreutils openssh-server openssh-clients
sudo systemctl enable sshd
sudo systemctl start sshd
sudo yum install postfix
sudo systemctl enable postfix
sudo systemctl start postfix
sudo firewall-cmd --permanent --add-service=http
sudo systemctl reload firewalld


Step 2:
curl -sS https://packages.gitlab.com/install/repositories/gitlab/gitlab-ce/script.rpm.sh | sudo bash
sudo yum install gitlab-ce

Step 3:
sudo gitlab-ctl reconfigure