Quick how-to on saving in Garry's Mod 13 since it isn't at all apparent, but it is quite simple.
First: Start a single player game.
Second: Open up your developer console (Default key is tilde "~"), if you don't have it enabled, then you need to enable it by going to your Options in game, "Keyboard", click "Advanced..." and check Enable Developer Console. Once you've done that, you can save by typing gm_save in console. After that you can go to the main menu, click "Saves" and find your saved game under "My Saves"
That's all there is to it. Hope this helps!