In the steam library rmb on the Gameguru icon on the left and choose properties. Middle tab bottom choice is the verify integrity of game catch option.
Right above that is the delete local game content. that will uninstall GG from steam, and you should be able to redownload and install.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit