I don't know how TES4Gegko behaves on Win7, but make sure you have the latest java libraries installed, and that HKLM\SOFTWARE\Bethesda Softworks\Oblivion exists in you system registry. Or reinstall TES4Gegko. Maybe you miss some files...
I think I found the problem. Since I am using 64BIT, the registry path is under wow6432node, not under SOFTWARE.
Fixed it, I simply made a new Bethesda key under SOFTWARE.