Garry's Mod
Garry's Mod (Source) Dedicated Server
Single-Threaded Source Engine
Source is single-threaded for game logic. Adding CPU cores won't increase player capacity. Reduce prop spawns and addon complexity instead.
Workshop Collection for Mods
To distribute mods, create a Steam Workshop collection of every addon your server uses and put the collection ID in the Workshop Collection startup variable. Players auto-download on first connect.
Steam Web API Token Required
A Game Server Login Token (GSLT) is required for the server to appear in the public browser. Generate one at steamcommunity.com/dev/managegameservers (App ID 4000) and set it in the Startup tab.
Official Documentation