Ideal operating system for running a dedicated server is Ubuntu or CentOS.
Ideal operating system for running a dedicated server is Ubuntu or CentOS.
Are you focusing on a solid desktop setup or just using it remotely via SSH?
If you're familiar with Ubuntu, choose Ubuntu—especially the long-term support version. It's more stable and doesn't require as many updates.
Whatever powers the software you require to operate. In the past (before the server caught fire), I used a very lightweight Lubuntu, with Xorg set to consistently mimic a 1366x768 screen for visual consistency. There were numerous scripts managing GMod tasks like creating instances, updating master files, and deploying patches to instances. Beyond the OS's memory consumption, you won't perceive any significant impact.