gmod is a sandbox game based on half life 2, gmod was made by a person named "Garry" the games full name is "Garry's mod" you can also mod the hell out of the game. It is only available on Windows 10+, macOS X and Linux, you can buy it on steam for only $6.69. Here is the link: https://store.steampowered.com/app/4000/Garrys_Mod/