Working with this data can be very challenging. We have been active in this space for twenty years and have worked through many of the ditches. For raw data you can go here:
Get in touch with a specific area of interest and we may be able to help out as we have post processed a lot of the GCM and CoRDEX data for a whole range of applications around the world.
Access to the base of these models is not possible. You can use the output of these models based on different scenarios of CO2 concentration on the basis of human activities. The outputs of each GCM model are presented in the website of that model (find by search the name of model), and also in some of other sites like the web site of IPCC.
If you follow the Global Circulation models links to Wikipedia, it will bring you eventually to the NOAA website where there is a page called NOMADS, which is a weather and ocean circulation model data site. It takes a lot of computational power to run these models. Good Luck with it.