The pilot, who was executing a Global Positioning System approach to a non-controlled airport with an unmonitored Common Traffic Advisory Frequency, saw a snowplow clearing snow from the runway as he broke out of the overcast in a light snow shower. Soon after the pilot spotted the snowplow, it exited the runway, and he continued his approach/landing sequence. After touchdown, the airplane began to slide when it encountered an area of ice and slush that still remained on the runway surface. Although the pilot decided he should initiate a go-around, he was never sufficiently realigned with the runway to feel that he could safely apply go-around power. As the pilot continued his efforts to realign the airplane, it departed the side of the runway, and prior to coming to a stop it impacted a natural gas line warning sign. The impact with the sign resulted in a tear in the stressed skin of the fuselage that exceeded eight inches in length. The pilot did not report any mechanical malfunction associated with the accident sequence.