Hmm... If neji can get into close range than he wins without a doubt, juuken can disable sand shield just like it disabled kidomaru's spider web.
The same for gaara's attacks, the sand has chakra and neji can stop the sand by disrupting the chakra flow the same way he stopped Kidomaru's webs with his juuken strikes.
Even if gaara got him in sand coffin, that would be useless because neji showed that he can release juuken chakra from all of his tenketsu points like he did, once again when Kidomaru trapped him in his spider webs.
Gaara has nothing to harm neji with, also, current Gaara does not have the ichibi, which rules out the possibilty of reopening his tenketsu points after they have been shut.
BUT, knowing about the hyuuga would probably take flight from the start and attack from long range, he may not be able to harm neji with his sand but he has more than enough chakra to outlast him, he is a former jinchuuriki afterall.
Neji won't be able to get close enough to disable gaara's armour with juuken and close his chakra points, neji wiil succumb to chakra exhaustion and die from sand coffin.
By the way if the only reason Neji isn't kage level is just plot no jutsu, the guy was stronger than naruto and sasuke and was like gaara level when they were genin, plus his genious level at the time was comparable to people like kakashi and imaybe even itachi.
A branch member who recreate kaiten and the 64 palms on his own was something thought to be unheard of and neji is revered even by hiashi as the man who has inherited the hyuga bloodline at a higher level than anyone before.
If Gaara is a kage level shinobi at the beginning of shipuuden than why isn't neji the same level? They were pretty much the same level in dubbed and neji is a genius prodigy, everyone else just got obsurd powerups, that's the only reason, and kidhi just didn't develop this guy for plot reasons.
