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I'm going to go trough each path and ability that Nagato has, and show you how Gaara can possibly counter all of them, and defeat a live Nagato WITHOUT GEDO MAZO. Not all of these points are concrete, and some of these are theories. Whether you agree or not is up to you.
1. Asura path - This is probably the least threatning path to Gaara. As long as he can keep his distance he poses no threat besides the missiles which can be disposed of by either blocking, or changing it's course with the sand. Whatever this path does is easily countered by Gaaras auto defense.
2. Animal Path - Each and every summon can easily be defeated by simply crushing them, the only one that would give Gaara some trouble would be the Multi-headed dog which he can seal. Also, because Gaara can attack with his sand in more places than one, and from the ground up the linked vision between the summons will be of no use.
3. Naraka Path/Outer Path - Pretty much useless in this fight, Nagato isn't pain, and has no need to revive any bodies.
4. Preta Path - As long as Gaara does not attack with the sand in his gourd Preta Path won't be able to absorb anything. The only sand that Gaara uses that is infused with his chakra is the sand in his gourd as stated by Deidara. Once he realizes that his chakra is being drained he can just use regular sand. This is the reason that Gaara without the Shukaku has been able to last so long in the war without tiring. Think about it.
5. Human Path - This can pretty much kill anyone, and anything if caught, but I don't think Nagato will be able to bring Gaara into his range. I'll explain this next.
6. Deva Path - Both Universal Pull and almighty push can be countered by Gaara grounding himself with the sand. He's done this to his opponents (Kimimaro, his Father, etc.), and there's no reason to say he couldn't do it to himself. It would be somewhat similar to how Tsunade was planning to counter Pain by concentrating chakra to her feet, but unlike Tsunade Gaara still has his auto-defense to minimize the effects of Shinra Tensei. He can use this same method to survive Chibaku Tensei. Gaara has been shown to be able to dig with his sand down to depths of 200 meters, probably even more now. If he were to latch onto his feet with his sand being that deep he's not going anywhere.
Side Notes:
- Nagatos biggest downfall in this fight is that he cannot move, if had the ability to move around he would destroy Gaara. I see him either being crushed or buried, heck, probably both.
- I'm not saying that Gaara would own him, or that he would win every time, but unlike many other characters in the manga he has a solid chance of doing it alone.
D) or not. Also, if hes not in the desert/ beach/ area with large amount of sand, I could see the fight going either way, or him exhausting himself alot sooner. It'd be interesting to see, and I'm happy someone out there doesn't just think ocular power = gg. 
