Gaara wins with mid-high difficulty. I believe Sasori will give him a good fight but Gaara should emerge the victor most of the time.
Gaara has more sand than Sasori has iron sand so despite iron sand being stronger, Gaara has more of it and is more versatile with it. Poison gas is going to be deadly although Gaara can enclose himself in a sand dome to prevent himself from getting poisoned or he can fly away.
Poisonous needles are useless. Most of Sasori's attacks are relatively useless against Gaara but Gaara's sand will creep into Sasori's various puppets and clog the joints.
Sasori's only chance is using iron sand to get through Gaara's regular sand. Even still Gaara has sand armour and has more sand to work with so he will eventually defeat Sasori.