try this

QuickTip quickTip = new QuickTip(grid);

grid.addListener(Events.OnMouseOver, new Listener<GridEvent<BeanModel>>(){

            @Override
            public void handleEvent(GridEvent<BeanModel> ge) {
                com.google.gwt.dom.client.Element el=  grid.getView().getCell(ge.getRowIndex(),ge.getColIndex());
                String html = "<span qtip='" + el.getFirstChildElement().getInnerText() + "'>" + el.getFirstChildElement().getInnerText()  + "</span>";     
                el.getFirstChildElement().setInnerHTML(html);

            }
});