Sand's weakness is fire and oily water. Point being, any master fire user can Katon all the sand Gaara uses into glass so he couldn't manipulate it. Furthermore, oily water puts a lot of weight on the sand Gaara uses so it would be very difficult to direct the sand anywhere. Lastly, the amount of sand Gaara can manipulate at once correlates to his chakra; even though Gaara is in a desert, it doesn't necessarily mean Gaara has the ability to throw the whole desert's components at once and call it a Sabaku Sozo. Being in a desert just means Gaara doesn't have to worry about running out of resources.