What operating system do you recommend for gameserver?

Versions of linux are generally the go-to. There are plenty of versions of linux, each with pros and cons. However the lighter it is, the more resources you have for the game itself. Windows uses plenty of memory for itself. However you might want to check if the dedicated server supports Linux.

Unturned Dedicated Server has an in-steam tool but for Garry's Mod, I suggest checking the wiki. I believe CS also uses a similar method, using SteamCMD