We dont know if it was the real Itachi or a kagebunshin which gave naruto the "gift", it happened just prior to the sasuke vs Itachi fight, and we all know Itachi planed to die at Sasukes hands. He also knew Madara would get to Sasuke after the fight, and that the outcome of their conversation could go against Konoha or the way Itachi hoped, against Madara... Because of this Itachi knew that if Sasuke would go against Konoha, only one would be able to beat or turn Sasuke around, he also knew that Naruto would need something extra for that fight. What that thing is we dont know, it could be all from the Sharingan it self, some way to put out Amaterasu, a way to stop Tsukyomi, or a hologram of Itachi him self with the true story about what happened in Konoha, with Itachi and his fealings.
The reason for the gift must have something to do with turning Sasuke against Madara, so that the two main characters of this manga, together, will defeat Madara at full strenght.
Well, thats what i believe....
