There are many plugins that are not part of the official gimp package.
They require to be compiled from C source.
For example: http://kcd.sourceforge.net/fix-ca.php
However, it is not possible to compile pugins for gimp.app.
See http://liquidrescale.wikidot.com/en:installation-instructions for explanation:
"the gimp.app project DMG distribution doesn't provide all the required headers etc… to compile the plugin".
Could you please add support for compiation of plugins (provide all the required headers, etc.)?
As a workaround, one can use gimp from MacPorts instead of gimp.app, but it takes a long time to compile all the required packages.