Generally speaking, you should not be using vendor specific prefixes. The W3C has recently been on the
war path on this
very issue. As such, I would not expect GWT and Google to officially support an officially unsupported CSS property.
Usually you can get away with these sorts of things with the literal() function ClientBundle to get around the unrecognized gradient constructor:
i.e.
div-with-literal { background-image: literal("linear-gradient(...)");}
Note however this does not help you get around an unsupported property name like -moz-transition. I'm not yet sure how to achieve this.
Sincerely,
Joseph