Instructions
Load "GMod" and enter a public game through the multiplayer menu, or enter a single-player game.
Open the console by pressing "~." If your console does not open, turn on the console activate from the main menu by clicking "Options," selecting the keyboard tab and checking the box for "Enable Developer Console."
In the console window type "bind" (without quotations), followed by the key that you want to bind in quotations. Type the model-changing command. Type the model name in quotations. For example, to bind the "y" key to model-change, type: bind "y" "cl_player modelname."
Press "Enter" and close the console by pressing "~." Press the bind key to change your model.
Repeat Steps 3 and 4, using a different key to bind for any number of models you want to use.