A GaugeField is a dual-purpose field that can be complicated to use. It is non-editable by default, but can be editable if the proper styles are set. Therefore, what it does and how it is used depends on whether the field is editable or not.
"Gauge" is a pretty generic term that doesn't clearly identify the purpose of the field. Instead, think of this as a merger between a progress meter and a slide bar control in Windows. When the field is not editable it is most commonly used as a progress meter, but when it is editable it functions as a slide bar control.
It is drawn as a thick, blue bar filling the remainder of the screen after the label portion. How much of the bar is filled in depends on the minimum, maximum, and current values. By incrementing the current value at regular intervals you can create an animation effect filling up the bar like the progress meter, and this is most commonly how it is used.
The minimum and maximum values can be any valid number so if you wanted to...