Not only does Gaara have the advantage in attack power, BUT he is protected from Neji's gentle fist. The gentle fist style lacks the offensive force to break Gaara's shield of sand. It can't even get past his Sand Armour, which is basically another shield.
Neji would not be able to hit Gaara without resorting to his strongest jutsu, and even then he wouldn't have the power to beat him.