If you can get any notification at all about the app resizing, you might be able to use that to calculate the new size for the Grid. For example (and probably overly simplfying, but hopefully it gets the idea across), you might know that whenever the browser window resizes, the grid should be 1/4 the size - 1/2 the height, and 1/2 the width. You might do something like this:
Code:
Window.addResizeHandler(new ResizeHandler() {
@Override
public void onResize(ResizeEvent event) {
myGrid.setPixelSize(event.getWidth() / 2, event.getHeight() / 2);
}
});