Georgia-Pacific is investing $70 million for upgrades to its plywood and lumber operations in Gurdon, Arkansas.   Georgia-Pacific bought the plant in 2006 from International Paper Co., which built it in 1979. That $237 million deal also included two other plywood and lumber complexes in Camden, Texas, and Springhill, La.; a plywood mill in Corrigan, Texas; and an engineered wood products mill in Thorsby, Ala.  Of the 2,400 workers at the five plants at the time, 650 were at the Gurdon plant, Georgia-Pacific said. Gurdon, in Clark County, has about 2,100 residents ...