<p>I'm running MAMP 1.7.2 on a Mac and I'd like to install the extension php_gd2.  How do I do this?  I know that on Windows using WAMP I'd simply select the <code>php_gd2</code> entry in the extensions menu to activate it.  How is it done when using MAMP? I know that I can do it using MacPorts but I'd prefer not to make any changes to my default OS X PHP installation. </p>

You shouldn't need to install the extension.  I have 1.7.2 installed and running right now and it has GD bundled (2.0.34 compatible).
From the MAMP start page, click on phpinfo and you should see a GD section.