Although Lee has become much stronger you have to look at who you're comparing. Gaara is Kazekage! The highest ranking ninja in the Sandvillage! It's not a tittle just anyone gets! Plus he has the ability to burry an entire forest in sand and crush anyone who gets caught in it. Lee may have the ability to open all 8 gates and move at near light speed, but it's only for a short time, and if he messes up, or is unable to get through Gaara's awesome defence, Gaara can spend the rest of the battle slowly crushing Lee into puddy. I just don't think Lee has the power to finish a battle with Gaara, exspecially when you look at how much Gaara has advanced since their last battle.