"GMod," which stands for "Garry's Mod," is a sandbox-style game that uses the "Half-Life 2" engine and gives you free rein over the game world -- this means that the gameplay is completely open and user-generated. You are allowed to spawn whatever objects you want and use them however they want. Bots are player-created nonplayer characters that act as placeholders for other players. They will not perform any actions unless directed to do so.
Load "Garry's Mod" and start a multiplayer game. It needs to be a multiplayer game since bots take up, and require, spaces normally delegated actual players.
Open the console by pressing "~" on your keyboard. If it does not open, go to the "Options" menu at the main menu and sleect the "Keyboard" tab. Click "Advanced" and check to "Enable Developer Console" and then click "OK." The console is the input box for all "GMod" commands.
While in a game that you created, open the console and type "sv_cheats 1"; hit "Enter." This will allow the use of more commands typically not enabled.
Open the console again and type "bot"; this will spawn a bot that will just stand in place. You can spawn as many bots as you want if space is allowed.
Make the bot mimic your movement by typing "bot_mimic" in the console.