You will need to create a Gamer Server Login Token (GSLT) to use your Garry's Mod server. Here is how to create that token:

Step 1: Click on the "Create GSLT" icon in your panel.

Step 2: You will need to login to your steam account. Once logged, you should see a screen like this, please enter 4000 for the game id and anything you want for the memo. Then press the Create button.

Step 3: If you receive an error, you will need to add a phone number to your Steam account via this URL: https://store.steampowered.com/phone/add

Step 4: Take note of the token that is provided as you will need it in a few steps

Step 5: Once completed, log in to your Host Havoc game panel.

Step 6: Click on the command line manager.

Step 7: Create a custom command line and in the section +sv_setsteamaccount add the token you were provided.- Save your command line and select it.