I am running the default setup and braking a little too much at even very slow speed the car wants to come around. I have to really brake like a have a raw egg under my foot. Even braking hard in a straight line the car wants to oversteer. I am running no aids at all. I have a track M3 and an Alpha Romeo Quad, and in real life nowhere near this problem with hard braking. This was in Montreal.
In general, the G55 is a little bit tricky on the brakes. You might want to adjust brake pressure, brake bias and close the brake ducts a little.
The G55 is sensitive to having cold tyres, warm them up for a lap and the problem tends to go away. Trail braking gently and for longer helps the car settle into a corner and doesn't unsettle the car as much as heavy braking, this is easier of course if you use left foot braking as well.
Just move the brake bias forward a bit, it it easiest if you have mapped buttons on the wheel for adjustment on the fly.