Therapy. Plenty of crazed people have become Sane through mental institutes.
Naruto was the light he needed to shine through the darkness he was entrapped in (no homo).
Uh, two problems with that.
1. Psychopachy cannot be fully treated in most cases. It can be managed and person suffering from it could lower the effectiveness of their psychopachy on them through therapy. So Gaara turning into a full on 'hero' who helps other out of his own willingless doesn't make much sense, especially since any kind of psychopathic tendencies are completely gone from him throughout Shippuden. Considering Gaara's shown some high-functioning traits worthy of a psychopath such as:
a) slaughtering people in cold-blood and a completely uncaring mindset to derive some pleasure for himself
b) extremely aggressive behaviour and no apparent empathy
c) talking to a non-existant, already dead person establishes how mentally unstable he was
2. Naruto saving him cannot have such a heavy effect on a mental disorder as psychopachy. It's not direct schema therapy which takes a notably long time to take it's effect on a psychopathic mind and even then it takes other methods to actually control and subdue psychopathic tendencies of the patient.
So yes, I'd just chalk it up to Kishi not knowing that he is basically doing an unrealistic psychopath apologist and complete cure concept that he's done with more than just one character and every character has successfully became more altrustic and gotten rid of their psychopathic tendencies.
