He's a kid that doesn't fully understand ethics and struggles with selfishness. He had very little social interaction since his aunt lived in the middle of nowhere and there weren't really any other kids to play with. A part of growing up and learning morality comes from social interaction and building relationships, something Gon only had with his aunt and.... well, animals, which don't teach right or wrong.
And considering the eventual "sociopathy" comes from *major spoilers*
calling it "sociopathic" is a misnomer. You can't care about people other than yourself and still be a sociopath.
Gon cares about Killua, Kurapika, Leorio, his aunt, so on.
That's not possible for a sociopath. It would be fair to call hin selfish, given that's always been a part of his character. Sociopath, on the other hand, indicates a poor understanding of the material.