For new installation of CentOS/RHEL, GUI doesn't come with default installation. If you do not click on the Software Selection link and pick server with GUI then there will be no GUI after reboot, only Base Environment will be installed. If the server is installed without GUI, please follow the steps below to install the required RPM packages.
$ yum groupinstall 'X Window System' 'GNOME' -y
$ systemctl isolate graphical.target
$ systemctl enable graphical.target --force
$ systemctl get-default
Hope this will help you.