Andrew,
GD comes with most versions of PHP, so you don't have to install it separately. If your Moodle variables page says GD is not installed, it means you need to install a newer or more complete version of PHP on your
server. For a Mac server, I suggest installing Marc Liyanage's build of PHP 4.3.6, available at
http://www.entropy.ch/software/macosx/php/
Let us know more information and we can help more.
Don