Hello
I'm not using Mac OS X, but I remember that the ports system is using Debian's APT under the hood and thought I'd mention the naming of the Gambit-C package in Debian (I'm not the maintainer, Kurt B. Kaiser is), for what it's worth:
gambc - Gambit-C Scheme interpreter and compiler gambc-doc - Documentation for the Gambit-C interpreter and compiler libgambc4 - Shared library for the Gambit-C interpreter and compiler libgambc4-dev - Header file and link for the Gambit-C shared library
http://packages.debian.org/gambc
Christian.