Noob question but how do I use game modes?

I wanna play some different game modes in single player but I have no idea how. They don’t have the option to select it when you play singleplayer any more or if they do, I can’t find it. I can’t find the answer anywhere. Will someone please help?

You download them and put the folder in gmod/gmod/gamemode folder, also not all the gamemodes you see in multiplayer are available, some you need to pay for.

That not want he asking, and to change gamemodes, you press start new game, and on the right side, there’s a tool icon, click it and go to the box to change gamemodes

Merci beaucoup.