The ADC and GPIO can be accessed in almost any version of kernels by means of direct access of the GPIO registers. The 2.6.30 and up has GPIO driver built to support easy access from user space applications. The GESBC-9G20 supports Linux 2.6.30 and up as long as you have correct patch set installed.
http://www.at91.com/linux4sam/bin/view/Linux4SAM/LinuxKernel has patches for different version of kernels.