We give you the tools and leave you to play. There aren't any predefined aims or goals. The original concept for GMOD was created as part of another game called Half-Life 2.Garry's Mod is a physics sandbox.
For a list of Garry's Mod console commands and a description of them, see the Console Command Category.There are to many to list here, but many other sites have already created a nice list for you.These are only a few common commands from the above lists. Some commands, such as changelevel will display a list of items related to the command (like a list of maps).Console Commands Console Command Category