"GAAANGG..."

This few days, my son like to say this word : "GANG..". I don't know what's the meaning of it or what that he refer to.
Last night conversation between Daddy & his son :

Daddy : "Nathan, call me .... "daddy"
Nathan : "Gaanngggg..."
Daddy : "No, not "gang", but "daddy"
Nathan : "Ganngggg"
Daddy : "Nooo... call "daddy", "daaa-ddyyy...."
Nathan : (pinch daddy's face) and say "Ganngggg..."
Daddy : "Aaawww.... !@#$%^&**@#"
Mommy : Hahahahaha.....