How to launch a custom gamemode on a local server?

I have a gamemode that I want to launch on a local server, but I don’t know how.
I put my gamemode in this folder: C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\gamemodes

It’s obviously not in the bottom right hand corner of my screen and when I type in console “gamemode arista-master” I get this error:
Couldn’t change active gamemode - ‘arista-master’ not found

Post a picture of what folders are inside your gamemodes/ folder and what is inside the folder of your gamemode, as well as the contents of your gamemodes .txt file.

Try arista

your gamemode folder must be named the same as the text file inside it. ( The folder is what the console command is looking for )

As for your text file, please look up at the correct examples in the sandbox gamemode ( or any other gamemode that is not base ) or here: http://wiki.garrysmod.com/page/Gamemode_Creation