Which GPower version is recommended for Windows 7 64 bit?
Check out http://www.gpower.hhu.de/en.html That page indicates version 3.1.9.2 for Windows 7 (32 and 64 bit).
The latest version of the software is compatible with XP, Vista, 7, and 8 (32 and 64 bit). To download it use the direct download URL posted below. Secondly, the official website offers the possibility to download manuals to learn how to use the application. Access the page to read more and to download those manuals.
Official website: http://www.gpower.hhu.de/ Download URL: http://www.gpower.hhu.de/fileadmin/redaktion/Fakultaeten/Mathematisch-Naturwissenschaftliche_Fakultaet/Psychologie/AAP/gpower/GPowerWin_3.1.9.2.zip Version tested: 3.1.9.2
Screenshot from Windows 7 x64bit: