What is GoogleWebToolkit?
I recommend you familiarize with GWT, before trying to use it with Openfire.
Integrating GWT into Openfire
Openfire makes internally use of Jetty, with provides an servlet container. So it is possible to run an GWT servlet with openfire. However, it’s a bit tricky.
I have written an minimal plugin as example. It uses AJAX to retrieve the current date and time from the server. My implementation has probably bad performance (*). If you come up with an better solution, please let me know.
(*) It’s synchronized for thread safety, so it can only process one remote call at once.
Obviously you will need a current version of GoogleWebToolkit.
You will need to copy gwt-servlet.jar from your GWT installation into plugins lib directory. It’s not included, because it’s “big” and part of GWT, so you have it already.
I supply two scripts for easy compiling client and server side code. Both are written in usual Linux Bash-shell. Nothing complicated, you will be able to translate it easily to your systems shell language. In any case you should take a quick look at it, probably you want to change the path to your GWT installation.
You can not licence your plugin under GPLv2, because GWT is under Apache License 2.0. GPLv2 is not compatible with Apache License 2.0. You could licence your plugin for example under GNU General Public License Version 3.