My Cooler Master V8 cooler cooler came with 2 different brackets.
One bracket was for Intel 775 as well as 1366 sockets. Only the fastening screw position changed. This was explained in the manual.
The other bracket was for AMD sockets, and I set this aside.
In your case, if you have the brackets and backplate for the 1366 socket (i7-920), then it is a matter of using the right bracket and fastening screws as suggested in the manual.
Here is what I found on Gigabyte's site -
http://www.gigabyte.com/search/search.aspx?kw=g-power+ii+pro
Extract:
G-Power II Pro
Fully compatible design, supports Intel LGA1366 / 1156 / 775 & AMD K8 / AM2 CPU platforms.
Curvaceous design combines CPU and MOSFET cooling.
12cm PWM smart-fan supporting high air bandwidth.
8mm, highly heat conducting heat pipes for enhanced cooling performance.
Pure copper base with aluminum fins, extra large heat dissipation area to dispose excess heat more efficiently.
Delicate design with blue LED light.
Only compatible with 200mm (or larger) width chassis....more
You are in luck! Take the motherboard out of the case, use the appropriate brackets and backplate, and start the installation per the manual.
I am assuming that you have done cooler installations in the past.
Good luck!