I can 100% guarantee that the automatic ban evasion detector will never be wrong, and we can look at the code together if you wish with other execs. The accounts must be connected in some way. This means his main account was banned at the time his alt account attempted to join the server.
EDIT:
OKAY so I figured out what happened.
You were banned for Mass RDM (your account is a family-shared account). The main account connected to the family-shared account you are using was also banned for Mass RDM, but it was never unbanned (so the server still thinks you're ban evading everytime you try to join).
Basically, I need the Steam ID of your family-shared main account that owns your copy of Garry's Mod so I can unban it. Then after that your shared account won't get banned for ban evasion every time. Otherwise, as long as your main stays banned, you will not be able to join the server.