How to Play GMOD Offlineby Alex Ramirez ; Updated September 22, 2017
"Garry's Mod" is a first-person shooter for the PC that connects to the Steam network every time you attempt to play it. If you leave the network, you will instantly lose connection and be disconnected without having the opportunity to save your game. If you are expecting to lose your network connection, you be preemptive and put Steam in "Offline Mode." This will allow you to play "Garry's Mod," even when you don't have an Internet connection.
Start Steam from the desktop icon or start menu. The program will boot to a login screen.
Check the "Remember by password" box and log into your account.
Launch "Garry's Mod" to see if there are any updates available. Once the game boots up, you may immediately close it.
Click on "Setting" from the "Steam" menu on the top toolbar.
Un-check the "Don't save account credentials on this computer" box, if necessary. By default, this is already turned off.
Open the Steam menu and click "Go Offline."
Click "Restart in Offline Mode," and you now have the ability to play "Garry's Mod" offline.