Instructions
Download the AlyxFakeFacotry NPC model.
Unzip the AlyxFakeFacotry NPC zip file using a decompression program such as WinZip, WinRar or UnRarX.
Copy and paste the files in the AlyxFakeFacotry NPC folder to your "addons" folder in "Garry's Mod."
Open the model menu in the game and spawn the FakeAlyx model. This model is invisible.
Select your model manipulation tool, then right-click on the FakeAlyx.
Reload your own model. You will now be invisible.