General Motors Co wants to boost the U.S. market share for its GMC truck brand by two-thirds over the next decade by advertising more heavily and introducing new vehicles, a top executive said on Monday. To help with its goal to boost its share to 5 percent from about 3 percent now, it is almost doubling spending on the brand in that time frame, including boosting its U.S. advertising budget by 50 percent this year, Duncan Aldred, U.S. vice president in charge of GMC sales, told reporters at GM's headquarters in Detroit.