They've already shown you how to get PHP-GTK code completion working in the Zend Studio IDE. In this new post they show you how to do the same with Eclipse running the PDT extension.

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

It's a simple eight step process (the steps are little) to get support working. You'll need to use the Gtk_Dumper to get the info the IDE will need for the completion.