To Quote Steam. "Garry's Mod is a physics sandbox. Unlike regular games there aren't any predefined aims or goals. We give you the tools and leave you to play." This Game is one of the best I have ever played. Second only to Dead space 2 in my book. Thanks to the community and the overall concept of the Mod. It adds an unheard of amount of Re-play ability. The game is constantly changing unless you don't want it to :O. One day you could be constructing a huge tower with automatic door and auto firing turrets. To making Machinima and Making random videos that are genuinely hilarious. *Ext. Link. 1, 2, 3.*
My Experience with this game was completely enjoyable. The Engine runs smoothly with little errors, in fact most errors are user made xD. (That says alot about how good at lua I am.) You can make your own weapons, models, Maps, cars anything you can think of. Watch this! If your really good at thinking you should try using Wire Mod it allows you to create auto opening doors and auto turrets.I would Suggest downloading the Source SDK.