With GWT, you can basically create any html structure you would like it to have by yourself. So it's not that hard to generate an accessibility act compliant webapplication.
What we did is hire an accessibility expert and have him define a fully compliant html with accompanying css.
His design encompassed all components that we are using (like table's, panels, li, tabpanels, popups, buttons etc etc..)
Our team then used GWT to have the components we need generate the html according to the designed html structure.
This also enabled us to use the same css as was defined. So now we can also outsource the css maintenance.
There were a few tuning issues (as expected), but now we (nearly) have a fully accessibility act compliant UI.
This was a little under 4 man month project. 2 months to get the accessibility expert up to speed and finish all components.
And 1-2 months to adapt the components we were already using to generating that html.
I'm not sure it is fair to expect the GWT project to generate fully compliant html.
The focus of GWT imo should be on the fact that you develop the UI in java, and have GWT compile java to working javascript.
That to me is the biggest USP and that should work flawlessly (as it always has).
What your application generates in html structure is up to you as user of GWT.