So Gaara couldn't sleep, because Shukaku was inside of him, hence his dark circle eyes. There was that guy who went crazy after trying to stay awake for 11 days for his science project. So how could Gaara function properly, let alone fight?
Evidence from Narutopedia
During their first conversation when Gaara was a child, Shukaku warned Gaara that it would take control of his body and start killing people if he ever fell asleep