A gaming server is a dedicated server used for online games such as World of Warcraft or Minecraft. The benefit of a dedicated server for gaming is that your server is often more stable than the game client itself, which is impacted by multiple players and resources. For a smooth gaming experience that you control, a dedicated server is the way to go.
The answer to this question largely depends on two factors. Are you new to server technology and not very interested? In this case, Windows is the easiest option to get where you want to go. Do you already have experience with Linux or would you like to familiarize yourself with this operating system? If so, Linux gives you more freedoms to configure your server, and you will be using an operating system that is less vulnerable to attacks from the Internet. Linux is also open source and therefore free.