This error is the easiest error to fix, try joining the server a couple times until you reach "sending client info" on the download bar

If the error persist, then go into your Garry's Mod settings, and under "Other" click the "When a game server tries to download custom content to your computer" dropdown box and select "Allow all custom files from server" or "only allow map files".

This error is annoying to fix, and you'll need to access Garry's Mod Folders.
This error occurs because a map file you already have is not the current version the server has, or at least in my understanding. An example of this is lets say ttt_rooftops_a2 is the server version, but your version ttt_rooftops_a2 (same name) has some extra content inside, this causes the error, and thats bad mapping if the editor doesnt rename their edited map.

1st Solution
The easiest way to access your local files is by right clicking Garry's Mod on Steam and clicking properties, then select the "local files" tab and click "browse local files".
In here reach your downloaded maps folder

Check the regular maps folder to make sure no files are downloaded in there.

skim through the list until you find the .bsp file and any files associated with the map, then remove it from that folder. Rejoin the server.

2nd Solution
You might've downloaded the map from the workshop, if the addon you downloaded has an updated .bsp, but still has the same name, then the error will occur.
First, go into Garry's Mod's workshop and search up the workshop map you downloaded, or search your subscribed items.
Unsubscribe the workshop item, to make sure the addon is completely removed, go into your Garry's Mod Addons folder

try rejoining the server.

Place all maps in here


TTT Server Pack
Here

Event Pack
Here

Place all maps in here


The Lobby
https://steamcommunity.com/sharedfiles/filedetails/?id=2363085906
Download the map through the workshop link, and any updates in the future will download the new version automatically.