Well Gaara and Naruto both have different strengths and weaknesses... Gaara's sand allows him to be farther away from a person and fight and gives him much more defense then Naruto, as Naruto's abilities are more hand to hand or brawling abilities... Also I remember that 1. Naruto was more focused on getting past Kimimaro and 2. Gaara and Rock Lee fought together and both couldn't defeat Kimimaro... his disease did that for them. Gaara being defeated by Naruto also doesn't mean that Gaara is weaker then Naruto. It just means that the two have different strength types.