GXT 4 with Material Design Font Icons as a ToolButton with Select Events that open up and Align Menu to them.

AppResources.java

import com.google.gwt.core.shared.GWT;
import com.google.gwt.resources.client.ClientBundle;
import com.google.gwt.resources.client.CssResource;

public interface AppResources extends ClientBundle {

  public static AppResources INSTANCE = GWT.create(AppResources.class);

  public interface Styles extends CssResource {
    String toolButton();
 
    String toolButtonOver();
  }

  @Source({ "styles.gss" })
  Styles styles();

}

MaterialButtonsWithAlignToExample.java

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.dom.client.Style.Unit;
import com.google.gwt.user.client.ui.RootPanel;
import com.sencha.gxt.core.client.Style.Anchor;
import com.sencha.gxt.core.client.Style.AnchorAlignment;
import com.sencha.gxt.widget.core.client.button.IconButton.IconConfig;
import com.sencha.gxt.widget.core.client.button.ToolButton;
import com.sencha.gxt.widget.core.client.container.FlowLayoutContainer;
import com.sencha.gxt.widget.core.client.event.SelectEvent;
import com.sencha.gxt.widget.core.client.event.SelectEvent.SelectHandler;
import com.sencha.gxt.widget.core.client.menu.Menu;
import com.sencha.gxt.widget.core.client.menu.MenuItem;

/**
 * Material Font Icons: https://google.github.io/material-design-icons/#icon-font-for-the-web
 *
 * Import: <link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
 */
public class MaterialButtonsWithAlignToExample implements EntryPoint {

  @Override
  public void onModuleLoad() {
    // only needed once
    AppResources.INSTANCE.styles().ensureInjected();

    ToolButton button1 = createMaterialButton("menu", "blue", 36);
    ToolButton button2 = createMaterialButton("more_vert", "green", 36);
    ToolButton button3 = createMaterialButton("error_outline", "red", 36);

    FlowLayoutContainer toolBar = new FlowLayoutContainer();
    toolBar.setBorders(true);
    toolBar.setHeight("38px");
    toolBar.add(button1);
    toolBar.add(button2);
    toolBar.add(button3);

    RootPanel.get().add(toolBar, 100, 100);
  }

  /**
   * Sizing: https://google.github.io/material-design-icons/#sizing
   */
  public ToolButton createMaterialButton(String iconName, String color, int size) {
    // define the styles for the button
    String toolStyle = AppResources.INSTANCE.styles().toolButton();
    String toolOverStyle = AppResources.INSTANCE.styles().toolButtonOver();

    // setup the tool button config
    IconConfig toolConfig = new IconConfig(toolStyle, toolOverStyle);

    // instantiate the button for use
    final ToolButton button = new ToolButton(toolConfig);
    // button.setBorders(true); // debug Size
    button.setPixelSize(size, size);
    button.addStyleName("material-icons");
    button.addStyleName("md-" + size);
    button.getElement().getStyle().setFontSize(size, Unit.PX);
    button.getElement().getStyle().setColor(color);
    button.getElement().setInnerText(iconName);
    button.addSelectHandler(new SelectHandler() {
      @Override
      public void onSelect(SelectEvent event) {
        fireSelectEvent(button);
      }
    });

    return button;
  }

  protected void fireSelectEvent(ToolButton button) {
    Menu menu = new Menu();
    menu.add(new MenuItem("Item 1"));
    menu.add(new MenuItem("Item 2"));
    menu.add(new MenuItem("Item 3"));
    menu.add(new MenuItem("Item 4"));
    menu.add(new MenuItem("Item 5"));

    // Align Menu to button
    AnchorAlignment alignTo = new AnchorAlignment(Anchor.TOP_LEFT, Anchor.BOTTOM_LEFT, true);
    menu.show(button.getElement(), alignTo, 0, 0);
  }

}

styles.gss

.toolButton {
}

.toolButtonOver {
	opacity: .5;
}