Ah, well, this is very hard. If it was the first season, gaara would so deffinatly win. Why??? Because one, neji isn't as expeirienced, two, gaara is already at a high level and has the demon inside him, and three, neji's destiny thing gets in the way of reality.
Now, if this were shippuden, then Neji would deffinatly win. Why??? Because one, neji becomes a WHOLE lot stronger, two, neji beat three S-rank akatsuki, three, gaara got beat by deidara while neji beat those THREE akatsuki, and finally, neji is faster and can easily go through the sand barrier that protects gaara.