I assume that you are referring to the Great Planes Gee Bee.

A friend, in the UK, has one, powered with a Magnum 1.20 4 stroke. It was assembled according to the instructions, with no balance issues. He is pleased with it's flight performance.

I have one, powered by a Zenoah G-23. I reinforced the firewall and moved the servos to the rear of the radio compartment for balance. I also placed a ply mounting plate on the firewall to get proper prop spacing. No additional weight was needed, but I did use a 1300ma battery pack to help out. I haven't flown it, as there is snow on the ground.

Although the wing loading will be higher, I foresee no issues with my setup.

I've included a few pics, that may aid you, if you decide to go with a gasser.