If the G-force is built to plan, doesn't the main airframe tube stick aft just a bit behind the fins (and fin tips)? And doesn't the motor stick out the back of that, even further? I would expect that you would have a lot of other banged-up stuff before you had much fin damage.
It's certainly your choice whether or not to reinforce, but I would think you might want to try building one stock and see how it works out. If it is a total disaster, you could always build another one later with stronger fins, but remember this: every once in a while, our rocket kit designers and manufacturers actually know what they are doing---the rocket might be just fine when built to plan.