Glad to hear you are OK Gnil!
It could have been worse... if the car would have caught with a curb or something and given sufficient inertia it could have flipped over and then you could have been seriouly injured. These things happen and could have happened to everyone especially with a GT3 and not having time to getting used to it.

No electronic stability, very grippy stiff tires which aren't very progressive (when they go they go quickly), very direct steering, instant-direct throttle, RWD only, etc... it all contributes to making it very difficult to control early enough when it goes without you expecting it (i.e. non-intentional and at the limit), and if you only had one hour with the car then you did well enough to avoid spinning and posibly smashing the rear (engine) and only hit the side.

The GT3 is an awsome trackday car, and great street car, but for twisty public roads nothing beats the -20mm rear diff, PSM equipped, CarreraS, and with your AWD even more manageable and effective. If it had been the other way around and it was your brother that grabbed instead your C4S-20mm, and did what you did, most likely nothing would of happened and probably would have been even faster with it than with his GT3 in that scenario.