I’m not sure about specifically GWT, but in JavaScript you remove the hyphens when using a prefix. webkit and ms are lower case, but O and Moz are uppercase. Here is some code for setting a transform in WebKit via vanilla JavaScript:
var el = document.getElementById("test");
el.style.webkitTransform = 'translate3d(100px, 100px, 15px) rotate(10deg)';
And here are the other prefixes and prefixless. Note the prefixless starts with lowercase:
el.style.MozTransform = 'translate3d(100px, 100px, 15px) rotate(10deg)';
el.style.msTransform = 'translate3d(100px, 100px, 15px) rotate(10deg)';
el.style.OTransform = 'translate3d(100px, 100px, 15px) rotate(10deg)';
el.style.transform = 'translate3d(100px, 100px, 15px) rotate(10deg)';
Here is a demo of it in action: http://jsfiddle.net/qScux/
EDIT:
In GWT, that'd be:
imageZoom.getElement().getStyle().setProperty("webkitTransform", "translate3d("+x+"px, "+y+"px, 0px)")