Garry’s Mod is a sandbox game. There aren’t any predefined aims or goals like in regular games. Players are given tools and are left to entertain themselves. Create your own games, or play games created by other users.
Overv has documented the 9 previous version in his epic news post over at ConstructiveNews.net. It’s an interesting read for anyone that was around at that time. It was especially interesting for me, since although I’d made all these versions I’d forgot about a lot of the things that are mentioned.