I suppose I'll put in my two cents.
So, Sasori's arsenal...
- Hiruko, Sasori's first line of both defense and offense.
- The Third Kazekage, giving him access to Iron Sand.
- Himself. Sasori is a pretty versatile weapon all on his own.
- A personal army of over a hundred other puppets at his disposal.
- Poison. On everything.
And Gaara's arsenal...
- The entire battlefield. Gaara has the home field advantage and the ability to strike from literally anywhere within those 30M.
- Shukaku, assuming this is before it was extracted.
There's no doubt that Sasori is incredibly deadly at both close and long-range, with all of his puppets, including himself, at his disposal, but Gaara has a single staggering advantage. His sand has the means of disabling most, if not all of Sasori's offensive capabilities. It's as easy as clogging up their mechanisms with sand and making them unusable, or at the very least much harder to use effectively. Even Sasori himself is susceptible to this. And with an entire battlefield of sand to manipulate for his purposes, where can Sasori go to avoid this? He's screwed on the ground, and not much better in the air.
In order of difficulty...
Hiruko, at least, is easily disabled, no diff. He's large and slow.
The Third Kazekage is a harder matchup, but is still low-diff. Iron Sand trumps regular sand, but unlike Sasori, who has a limited amount, Gaara has a whole thirty meters worth of sand to contest the Kazekage puppet's. He could easily keep Sasori on the defensive sheerly by overwhelming him. Sakura destroyed the Kazekage puppet by reading its movements in the way Sasori was manipulating it. How hard could it be for Gaara to do the same, without ever needing to get close enough to be threatened by poison.
Sasori himself is a mid-diff threat. Because he is himself, there's no inherent lag in his movements like there would be for the Kazekage puppet, which makes catching Sasori more difficult, but still very possible in a battlefield of sand.
Sasori's hundred-puppet army is a much greater challenge in this matchup, but I still think Gaara pulls through, high-diff. A single Sand Tsunami invariably takes out at least several puppets. Sasori's close range puppets put Gaara on the defensive, but they could still be methodically disabled. If worse comes to worst, letting Shukaku loose is a last resort.
If it was anywhere else, like a forest or plain, Sasori takes it mid-diff at best. But putting them in the Sand Village gives Gaara an insurmountable advantage. There's a reason Sasori let Deidara go in alone to capture Gaara.
