Impulse is the successor to SDC which has been the only way to patch GC2 since the 1.4x update in late 2006. If you already registered the game previously, it is as simple as:
- Install Impulse.
- Run Impulse and log in with the Stardock/forums account that you registered the game to.
- Go to the My Games tab, pick GC2, and click the Update/Install button.
If you did not previously register it, all you need to do is click the Register Product button at the top of the My Games tab in Impulse and enter your email/serial key.