Lightweight desktop environments to replace GNOME for CentOS?

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:

• XFCE
• Mate
• LXDE
• LXQt
• etc.

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.

There is a lot of "lightweight" Desktop Environments and Windows Managers available to all Unix-Like Operating Systems. They are good for not being resource hungry, for having a bunch of basic tools for daily usage(terminal emulator, file manager, music player, menus, etc) but they can't operate miracles on hardware that is way too dated.

Some of the desktop environments are:

• LXQt - A Qt based desktop, result from the merge of the LXDE and Razor-Qt interfaces merge.
• XFCE - A Gnome'ish looking desktop focused on stability
• Mate - A "Gnome 2 Fork", that tries to be as simple as Gnome 2 was.
• Cinnamon - Same as Mate, but officially endorsed by Linux Mint

I'll not suggest tiling Window Managers here like i3wm cause, even if they are less resource hungry than any of the options above, they are "bare metal" and need a lot of customization/tunning to turn them usable. And since they are tiling, it can cause confusion on some users cause they are pretty intense on keyboard shortcuts using.