Given your use case, and if you are this low on memory, the best choice would be to switch to a lightweight Desktop Environnement (DE), such as:
If you are (even sort of) new to Linux, I would suggest you to stay away from Tilling Window Managers (TWM); although being extremely lightweight and powerful once configured and mastered, I do not think one of these would be a good idea given your situation.
If you want to install XFCE (example):
First, you need to add the Extra Packages for Enterprise Linux (EPEL) repository, as this is where you will install packages from:
# yum -y install epel-release
Then you can install XFCE Desktop Environment as following:
# yum -y groupinstall X11 # yum -y groups install "Xfce"
After a reboot, you will be able to switch to XFCE4 instead of using GNOME3 at the login screen.