I've replied to this about 10 times in the Primavera thread
He and his brother signed but they can't register to play until January 2012 because of their age/paperwork before the last window closed.
He's trained with the first team and played in a recent friendly, word is everyone is impressed, talking about Gabriel as the vice-Pirlo. We'll see, he's only 18, but he does look very assured and has a class left foot.
I'm not sure about his brother, seems a bit hit and miss from the videos, probably another Chedric Seedorf