a single car won't do anything to gas prices. Gas prices are set on the global market. China and India have a combined 2.3 BILLION people and they are all starting to buy cars. The US has 300 million people. It won't make a lick of difference in gas prices.
It IS a step in the right direction. As they say, a journey of 1000 miles begins with a single step. The Volt is one of the first steps in a long journey.
As far as whether or not it will help GM? Probably not. The car is expensive to produce, they won't be making any money from it. GM's financial problems run deep.