Garry's Mod Server Hosting

Any of them. Sandbox is the default and Trouble in Terrorist Town ships with the game, so both are a single panel setting. DarkRP, Prop Hunt, Murder and the rest go into the gamemodes folder over FTP or arrive with your Workshop collection, and then you enter the gamemode name in the panel.

Make a collection on Steam, copy the ID from its URL and paste it into the Workshop field in the panel. The server downloads everything in it at the next start. For players to receive the same addons automatically when they join, add one resource.AddWorkshop line per addon in a Lua file under lua/autorun/server.

Almost always because the map or an addon uses Counter-Strike: Source assets. Players need the CS:S content on their own PC, and if the server needs it too, upload the content over FTP and point to it in cfg/mount.cfg.

No, the server starts without one. A token from Steam's game server account page ties the server to your Steam account, so players who keep it in their favourites still find it if the IP address ever changes. There is a field for it in the panel.

The panel default is 22, which keeps CPU use low and is fine for building and roleplay. Shooter style gamemodes such as TTT feel better at 33 or 66. A higher tickrate costs more CPU, so raise it in steps and keep an eye on the server load in the panel.

Plans start at 2 GB, which covers Sandbox or TTT with a modest addon list. DarkRP with a big collection and a busy map wants more, and you can move up in 2 GB steps to 8 GB without losing anything on the server.