Orochi should win more times then not. With him being able to blend into the surrounding he can pretty much avoid damage Gaara might be able to deal to him if he uses it. If Gaara manages to catch him throughout the fight then Orochi can always use Manda to break him out of said sand bind before he gets crushed upon which Manda can help overwhelm Gaara as well. Add in Yamata plus his other snakes along with his sword and Orochi should be able to dance around Gaara's sand long enough to overwhelm him.
Gaara has an advantage being in the desert, but Orochimaru still beats him anyway.
Orochimaru can hide and evade attacks by melding into the environment with Leech All Creation, or by using large snakes to burrow under the sand or plow through it. If he’s captured by sand, he can escape with a preemptive Oral Rebirth, as long as his entire body isn’t already enveloped. In the event he is completely covered, he can summon a giant snake and break out similar to how Naruto used summoning in Part 1. For small or moderate amounts of sand, Orochimaru should be able to blast them away with Wind Style as well.
Unfortunately for Gaara, his main defense — sand shields — would be ineffective against Orochimaru’s Sword of Kusanagi. With Kusanagi: Longsword of the Sky, Orochimaru can punch straight through giant waves/walls of sand to skewer Gaara. Coupled with underhanded tactics such as Shadow Clone feints or an underground attack after a Leech All Creation, and it’s hard to see Gaara successfully evading, especially if his attention is already divided trying to ensnare fast-moving snake summons.
It’s not needed, but if Orochimaru feels hard-pressed, he can assume his Eight-Headed Serpent form. Its size and power should allow it to shrug off and smash through tidal waves of sand attempting to restrain it, and it can overwhelm Gaara by attacking from multiple angles at once thanks to its many heads. While Gaara is busy trying to fend off several thrashing heads, Orochimaru can take advantage and snipe with Kusanagi: Longsword of the Sky.
Even in the desert, Orochimaru has the tools to win, and he’s just the stronger fighter overall. Impure World Resurrection is not needed here, and if the zombie Hokage are summoned, Gaara is going to get quickly overwhlemed, as he cannot deal with giant snakes, Orochimaru’s Sword of Kusanagi, and Birth of Trees or Bringer of Darkness at the same time.