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If you are this low on memory, the best choice would be to switch a lightweight Desktop Environnement (DE), such as:

If you 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 this 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

# sed -i -e "s/\]$/\]\npriority=5/g" /etc/yum.repos.d/epel.repo # set [priority=5]
# sed -i -e "s/enabled=1/enabled=0/g" /etc/yum.repos.d/epel.repo # for another way, change to [enabled=0] and use it only when needed
# yum --enablerepo=epel install [Package] # if [enabled=0], input a command to use the repository

Then you can install XFCE Desktop Environment as following:

# yum -y groupinstall X11
# yum --enablerepo=epel -y groups install "Xfce"

After a reboot, you will be able to switch to XFCE4 instead of using GNOME3.