If you are playing in a Garry's Mod server that has the addon "precision tool", you can apply the tool to a prop (i use a chair) and get in quick to fall through the map and reach the skybox. First, click on the precision tool. Then, click "< General Tool Options >". Three checkboxes will appear. Click 3. Experimental. After that, deselect everything besides "Only collide with player" in the checkboxes above "< General Tool Options >." So all you have to do now is apply the tool to a chair and get in quickly to fall through the map. Good luck!
