In order to add a (GNOME) desktop to CentOS 7 issue the following commands in your (elevated) terminal:
- yum groupinstall "GNOME Desktop"
- unlink /etc/systemd/system/default.target
- ln -sf /lib/systemd/system/graphical.target /etc/systemd/system/default.target
- reboot the system
After the reboot you should now face a working GUI environment...