.marginRight{
 margin-right: 10px;
}
FlowPanel myFP=new FlowPanel();
Button myButton=new Button("Button");
Button myButton2=new Button("Button2");
myButton.addStyleName("marginRight");
myFP.add(myButton);
myFP.add(myButton2);

<div><button class="gwt-Button GHKUF0UDD4" type="button">Button</button><button class="gwt-Button" type="button">Button2</button></div>

.marginRight{
    margin-right: 10px !important;
}

myButton.setStyleName("marginRight");