QTP with the Web Add-In does indeed work with Ext GWT. Of course, because even the simplest of EXT widgets are complex UI controls made up of multiple generic WebElements, using Record & Playback will not get you very far in interacting with and validating these controls.
We've implemented a library of functions specific to supporting ExtJS and Ext GWT controls as part of the overall generic framework we use for validating our web-based applications. That library currently supports interaction with, and validation of Ext comboBoxes, grids, panels, windows, form edit fields, tool tips, and menus.
Our biggest challenge has been updating the library functions each time a new version of the Ext GWT controls are released, as our functions make extensive use of the innerhtml and outerhtml properties of the various WebElements that comprise a single Ext widget.