There is only one patch (against gcc 3.2(.3)), you use gnat 3.15p to bootstrap the compiler.
Check the ebuild for the complete build process (dev-lang/gnat). As for gcc cross compilation,
if you are not already familiar with it, there are lots of sites describing this. For instance
the gcc cross compilation faq.

//David Holm

On Mon, 01 Sep 2003 19:47:31 +0100
"Luke A. Guest" <laguest@abyss2-nospam.demon.co.uk> wrote:

> On Mon, 01 Sep 2003 03:19:10 +0000, David Holm wrote:
> 
> > I have never cross-compiled gnat, but afaik it wouldn't differ from how you cross compiled
> > C/C++. Are you looking for plain mips-elf binaries or do you need it on Gentoo-mips? I will
> > port gnat to a couple of platforms when I can find the spare time. It would be nice to know
> > which platforms people use it on (I only have x86 and ppc).
> 
> Plain, it's for my own thingy. Basically, I need to know which patches (if
> there are any) and the order of application.
> 
> Ta.
> 
> Luke.
>