The programs generated by GWT are web applications (GWT makes it possible to develop the frontend, i.e. the user interface for a web application, in Java), so you can use a GWT app anywhere.
The developer tools for creating a GWT app are Eclipse, an Eclipse plugin, a Java SDK, and some browser add-ons for testing. I don't know if the add-ons for testing work on AIX, but everything else should. The developer tools are otherwise supported on Windows, Mac, and Linux.