Update: Version 3 is now out! If you came directly to this page and haven't tried the new version yet, scroll up and click on the Downloads page for the mod. The Reversal mod is a complete balance and faction upgrade for Ground Control 2, fixing numerous balance problems and enabling the Terran faction for play in single and multiplayer modes, including the campaign.
Patch your game to 126.96.36.199. Unzip all files in this .zip folder into your main Ground Control 2 directory. You will need a program such as WinZip or similar. There are three .bat files included - one for each faction combination. Choose which combination you want to play on and run the .bat file for it.
There are more details in the readme included in the .zip and in the content posts here on moddb. Enjoy.