I think Gaara would have the upper hand when fighting with Sasori.
Gaara has so many techniques that can block Sasori's puppet weapons.
Like:
You must be registered for see images
You must be registered for see images
Also, Gaara can avoid the attacks because of his flying sand Nimbus whatever
You must be registered for see images
But I also think Sasori can counter these defenses with his 3rd Kazekage's puppet's irons sand to cut those defenses.
You must be registered for see images
But then again. Gaara can harden his sand with the nutrients in the soil as mentioned in his fight with Kimimaro Which can also block
Sasori's Iron sand attacks.