It looks like GeoMaps does not include AT-9 in its map. In your luck, GeoCharts (the non-flash newer version of GeoMaps) does.
I don't use GWT, so I won't be able to tell you how to translate this in to GWT (apologies, maybe someone else can tack on the proper syntax). Here is the code:
function drawVisualization() {
var data = google.visualization.arrayToDataTable([
['Bundesland', 'Sessions'],
['AT-3', 200],
['AT-5', 56],
['AT-7', 11],
['AT-4', 11],
['AT-1', 55],
['AT-8', 567],
['AT-2', 11],
['AT-6', 99],
['AT-9', 1]
]);
var geochart = new google.visualization.GeoChart(
document.getElementById('visualization'));
geochart.draw(data, {region: 'AT', resolution: 'provinces', width: 556, height: 347});
}
In general, I suggest using the ISO-3166 codes for whichever country you map to make it render faster. Let me know if it works!