From what I have discovered I don't think there is a specific GTI manual.
I've been told that the Haynes Manual called the Owners Workshop Manual (Green cover) is the better resource to have then the Haynes Service manual (Red cover)- which doesn't have as much info.
The other publication is the Bentley Service Manual, which is written for US models - which although is not totally relevant to us, still does have some useful and relevant info.