How do I change the gamemode for my server?

I have downloaded several game modes, but I cannot figure out how to play said gamemodes. Can someone please help?

Put the gamemode file into your garrysmod/garrysmod/gamemodes folder and go into Garry’s Mod and click start new game >> then click the tool icon to the right of the map menu and under “Default” change it to the newly installed gamemode, hope this helps.