The Gun Boat is a Quad fifty mount for the Half Track - a very good weapons system for pinning down soldiers. At the cost of 15 TP, it's relatively cheap, too (compared to 4 fifties at 24 TP). At higher Rifle skill, it is a more resilient and deadly version of a Gunner, being able to deal death to anything that enters the battlefield, as long as it isn't heavily armored.
The M45 Quadmount was a mobile anti-aircraft weapon used during World War II. It was sometimes mounted on the M3 Halftrack for greater mobility, the combined weapon being known as the M16 Multiple Gun Motor Carriage. The Quad .50-cal was also reffered to as the 'Meat Chopper.'