As you probably know, PCLinuxOS is a KDE based distro. Although there is a GNOME site (sub-community) called Linux Gator, it seems that GNOME is still using version GNOME 2.32. You can download GNOME ZEN Mini to test GNOME, but as you can see, the ISO is more than a year old (January 2012). So if you want to download and install it, change your repositories from the list. On the other hand, MATE desktop has a lot of bugfixes and new features. So you better install and use it.
If you don't have PCLinuxOS installed, you can either download the GNOME version or download some MINI version from the site http://www.pclinuxos.com (eg for LXDE, download from Finish server).
After doing all updates, you should add the following repository:
rpm http://repo.pclinuxos.su/rulinuxos/apt/ etc/i586 mate
How to do that? The easiest way is to open Synaptic and add it there, as the picture shows:
Reload the repositories, and search for mate (or task-mate). Mark it for install and hit the apply button. After installation, log out and then back in, choosing MATE as your session. From now on you can see:
There are also some rpm packages:
Also you can download the ISO (it's Russian, but you can change it to your language).
MATE-desktop для PCLinuxOS