systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.
The key commands or arguments you will use with systemctl are the following:
list-units [PATTERN...] List loaded units
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"