The PHP-GTK Community Site has posted about enabling the code completion for Zend Studio to work with the PHP-GTK specific functionality.

Enabling PHP-GTK2 code completion in Zend Studio 5.5 for a project is very simple. The following example is given on Windows, but the steps should be the same on any supported platform.

Screenshots are also included in the step-by-step process. You'll need to run a bit of code to build the completion file, but after that it's just a few quick clicks away.