Step 1: Navigate to the Game Panel and Login
Step 2: Navigate to this Pathway in the File Manager (/home/container/garrysmod/cfg/server.cfg)
On a new line in the server.cfg file, add the line sv_loadingurl "YOUR URL HERE"
Replace the "YOUR URL HERE" with the URL of the page you would like to use for your loading screen.
Step 3: Save Your server.cfg And Restart Your Server