General Motors Corp. is creating a new product-development and marketing strategy that calls for introducing an all-new vehicle every month by 2000, says Roy S. Roberts, general manager of GM's Pontiac-GMC Div. He also says GM will increase truck production from 40% of total capacity today to 50% by 2000 to meet the growing demand for trucks in the marketplace. "A product a month, every month. We're not concerned about ail the cars coming out in the fall. We're going to bring the product out when the product is ready and we're going to drive excitement into our dealerships year-round. That's our goal," Mr. Roberts says. "it's not a slogan, not a fad. It is a hard-line plan."