For a gauge on the dash, it's most likely connected to a sending unit in the to cooler circuit. Usually, it's in one of the big fittings at the transmission. Those are installed by the chassis manufacturer, and may or may not be an Allison part.
Look for the two large lines going to the front of the transmission. Like was said the normal place for the sender is in a Tee on one of the large lines at the transmission. The sender is looking at the oil temperature before it is sent to the cooler.