Taiwan said its gross national product growth would fall to 5 to 6 percent annually in the first half of 1985, well below previous expectations and last year`s levels. In the first six months of 1984, Taiwan`s GNP grew at a 12 percent annual rate, and the government had predicted an 8.5 percent growth rate for the first half of this year. Government officials attributed the drop to slowing export sales and a slight decline in foreign investment in Taiwan.