Cossacks I / American Conquest .GP Regenerator.
This tool can batch fix extracted .gp files to restore the national colour and alpha channel.
Run the Setup.exe file to install.
On newer computers ( Vista, Windows 7) it is possible that will ask you to install netframework version 1.1.4322.
This version is included in the rar. Run dotnetfx.exe to install and then run the setup.exe again
Special thanks to [TFN]SilverSurfer for finding this long lost tool back.