The hard way

MyViewImpl.java

public class MyViewImpl implements MyView, AttachEvent.Handler {
    interface Binder extends UiBinder<Widget, MyViewImpl> {}

    private final Widget widget;

    @Inject
    MyViewImpl(Binder binder) {
        widget = binder.createAndBindUi(this);
        widget.addAttachHandler(this);
    }

    @Override
    public void onAttachOrDetach(AttachEvent event) {
        if (event.isAttached()) {
            Document.get().setTitle("This is the new title of the page");
            updateMetaTags();
        }
    }

    private void updateMetaTags() {
        injectMetaProperty("description", "This is the description");
        injectMetaProperty("og:image", "http://placehold.it/350x150");
        // ...
    }

    private void injectMetaProperty(String property, String value) {
        Document document = Document.get();
        MetaElement metaElement = document.createMetaElement();
        metaElement.setName(property);
        metaElement.setContent(value);

        document.getHead().insertFirst(metaElement);
    }

    // ...
}