This video will take you through all the steps of setting up a gmod dedicated server.
Code for UPDATE.BAT:
cd C:\Gmod13server
hldsupdatetool -command update -game garrysmodbeta -dir C:\gmod13server
Install link for Server Checker: http://www.mediafire.com/?ej1mzzxgnnr
Text to insert into server.cfg:
hostname “Garry’s Mod 13″
rcon_password “”
sv_lan 0
Duration : 0:6:25