Garry's Mod is the best thing ever. It is not a game. It is more of a multi-gaming platform where you have lots of GameModes all differentiating from each other by their unique styles and flows. DarkRP, which is the most popular gamemode gives you the oppurtunity to start your own life and have lots of fun messing around, purchasing cars, land and guns, building your stuff. TTT, which is also very popular is a parody of CS: Source and it is very fun, it requires strategic thinking abilities and also you need to have good reflexes. As for Murder it's plain, exciting and fun, you never know who's going to take out a knife and stab you before you can say anything!