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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.

login screenshot