At least you must know how to build gwt app,it's better to know what is gwt-gadget.
from Google Drive
use Hello-hangouts.zip and modify it.
akjava_gwt_hangouts1.0.jar(GWT Hangout wrapper 1.0)
gwt-gadgets_hangout_custom_by_akjava.com.jar(gadget.jar which customized for hangout)
If you are not familiar gadget copy and paste (SimpleGadget.java & Simple Feature)
extends SimpleGadget and overwrite onApiReady(),and edit ModulePrefs
create com/google/gwt/gadgets/linker/computeUrlForGadgetResource.js and modify for your server
This is very important,if you are not modify it,you can see such kind error when you load a app.
you must replace url to in where your app is uploaded