http://mygshock.com/ is a great place to get periodic updates on new releases and other developments in the G-Shock world. Riley, who runs the site, has been a fixture in the G-Shock community for many years.
The watch archive, at watchshock.com, is also an invaluable resource, particularly for older models. Like all such resources, it is not perfect, but is has a well organized wealth of information and photographs of a huge number of G-Shock models.