Summary
Garry's Mod servers, like most source game servers, require an AuthKey or 'GSLT Key' to run publicly. This is so that there is a layer of accountability for server owners to ensure that real people create them and to hold them accountable for any violations of the game's terms of service. There are multiple qualifications for generating a GSLT key; the user must not be banned or 'limited' and must have a registered phone. In addition, the user must own the game and have a cap of a thousand game server keys.
How to Generate a GSLT Key For a Garry's Mod Server
1. Log in to the Steam Community website.
2. Go to the Game Server Account Management page.
3. At the bottom of the page, enter 4000 for the App ID to designate Garry's Mod as the game.
4. Add a memo to distinguish between tokens easily, then choose Create.
5. Copy the Login Token that appears; this is the GLST key.
6. Log in to the BisectHosting Games panel.
7. Stop the server.
8. Go to the Startup tab.
9. Navigate to the Authkey section. 
10. Paste the Token into the field and toggle the key On.
11. Start the server.
