Did you just install the minimal version by accident or want to install the GUI? No need to reinstall just tell yum to do the work for you!
You could actually have both installed and choose one as your preference on demand (although many people prefer GNOME's simplicity).
Install Gnome and/or KDE.
To install the GNOME Desktop on Centos 7:
yum -y groups install "GNOME Desktop"
To install the KDE Desktop on Centos 7:
yum -y groups install "KDE Plasma Workspaces"
Type the following command (no need to reboot!):
Now you'll be into your graphical session!
Make it permanent otherwise you will start in run level 3 without any GUI after rebooting:
systemctl set-default graphical.target
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