gtkada install

[M]

Hi,
First time MacPorts user.  I installed MacPorts 1.9.2 on Mac OS 10.6.5 (Xcode 3.2.5).  First port I wanted to install was gnat-gtkada.  I ran: "sudo port install gnat-gtkada".  Everything went fine until I got this message:
#  Ada is self hosted (http://en.wikipedia.org/wiki/Self-hosting)
#   
#  You need to install an existing Ada compiler and then choose
#  an appropiate variant. For more info use: 
# 
#  port variants gnat-gcc

I already have GNAT & AdaCore's GPS installed (20100603).  Not knowing how to tell MacPorts that GNAT is already installed, I tried this: "sudo port -v install gnat-gcc +gnatgpl", and got this message:
#  Ada is self hosted (http://en.wikipedia.org/wiki/Self-hosting)
#
#  Bootstrap compiler not found. Please download and install from 
#  http://libre.adacore.com

That's what I've already got installed.
How do I tell MacPorts that I have GNAT installed (and is already in my command path at /usr/local/gnat/bin), so that I can get gtkada installed.

Thanks