"Garry's Mod," also known as "GMod," is an online sandbox physics game for Windows and Mac-based computer systems that uses assets from games developed by Valve Software, such as "Half-Life 2," "Left 4 Dead" and "Team Fortress 2." "GMod" allows you to spawn objects and characters and use special tools to manipulate these objects and characters in various ways, such as creating a thermonuclear catapult or making the characters of "Team Fortress 2" perform a can-can dance. You start "GMod" as a basic scientist character, but can change your character through the "GMod" options menu.
Press the "Q" key while playing "GMod" to open the main "GMod" menu. Click the "Options" tab in the upper-right corner of the main "GMod" menu to open the options menu.
Click the "Model" entry under the "Player" header of the options menu to open the character selection menu. Click on a character model to select that character, then press "Q" to close the main "GMod" menu.
Kill yourself in "GMod" to effect the character change. You can kill yourself in a variety of ways, including dropping a heavy object on yourself, running in front of a vehicle or other fast-moving object, detonating an explosive object while standing next to it, or by using the "kill" command in the "GMod" command console. You will re-spawn as your selected character after dying.