Garry’s Mod is a sandbox mod for the Source Engine. Unlike normal games there aren't any predefined aims or goals. Players are given tools and are left to entertain themselves. The player is in a world where objects can be created and destroyed on demand. Where people can be created and destroyed with the click of a mouse. A world made for him. A world he made.
Do you mean your Hugal's sister?
yup
What is wrong with those turrets?
Some kind of disease. Something humourously similar, I don't know any relevant diseases.