Hi Michael, we do this using State Formatting. If you need an overview on that, you can go to Support/Training and go to the Visualizations video.
A widget can only be state formatted based on values bound to that Widget. So you could do two things since this is a gauge.
One create a service that appends that true/false to the value you are displaying in the gauge and use it that way to do the state based formatting.
The other way is to overlay another widget onto the gauge that is tied to the value of the service and state base format that widget.
