

It’s not CentOS 3, it’s CentOS with Linux kernel 3.10 (a 2014 kernel). This was supported in RHEL/CentOS through 2017.
Still very dated and a bad idea, of course. And even weirder that it’s on a new machine. I’ve seen tons of stores using Win7 past it’s EOL, but on older hardware.
The distros such as Debian, Fedora, Arch, and Ubuntu make all kinds of DEs available to the user to install. Gnome is not in charge of this, and even if they were, the suggestion that they would make other DEs unavailable is childish. You have plenty of choices