3) Have Garry's Mod installed. If you do not have it, it is 9.99€ on Steam.
4) Once these three conditions are met, you can join any server from the list of servers on the website by clicking on the "Connection" button.
You will get a download if this is your first connection. This is the necessary content that installs automatically. Then the loads will be much faster.
📌 2. Do not have a cheat installed and activated
- If cheating software is installed, this is not trivial, possession of cheating software can be a valid reason for permanent ban, uninstall them before joining the servers. It's a real wound on Gmod.