GPGnet Full Installer v1.5.225http://thevault.gaspowered.com/gpgnetdo ... .5.225.zip
First, unzip the contents completely. Then run Setup.exe (Not GPG.Setup.msi or GPG.Setup).
If you are in a situation where you are only trying to reinstall GPGnet and not Supreme Commander, please make sure you have done the following;
1) Uninstall GPGnet via the Windows add\remove programs option.
2) Navigate to the c:\program files\THQ\Gas Powered Games directory and remove the GPGnet folder.
Vista users, after install, make sure you run the GPGnet client as administrator the first time.