After installing CentOS 7 you need to execute the following commands in order to install Gnome, or in other words have GUI mode of CentOS.
sudo yum groupinstall "Gnome Desktop"
startx

Thou shalt programme
After installing CentOS 7 you need to execute the following commands in order to install Gnome, or in other words have GUI mode of CentOS.
sudo yum groupinstall "Gnome Desktop"
startx