Gnome is a whole graphical desktop based on the gtk toolkit. It was the nest for the AT-SPI facility, and hence is pretty much accessible nowadays.
There are two useful HOWTOs: [http://library.gnome.org/devel/accessibility-devel-guide/nightly/ one for developers], and [http://library.gnome.org/users/gnome-access-guide/nightly/ one for users].