This server runs a custom build of Jazztronauts, so the Steam Workshop version will not work with it. You need these files instead:
steamapps/common/GarrysMod/garrysmod/addons/jazztronauts/ containing a gamemodes folder.password THEPASSWORD; connect jazz.multiversal.nexus (ask whoever invited you for the password)Also recommended: have Half-Life 2, Episodes 1 & 2, Counter-Strike: Source and TF2 installed and mounted in GMod, so stolen maps aren't full of missing textures.