Gai wins with ease. He handled many sound & Sand fodder of jounin level... Ten Ten would just be gennin level fodder (she is only Chunin b/c of her teammates strength)... Gai wouldn't even break a sweat.
Tenten is a chuunin by her own strength. I believe that 100%. She's not gotten any combat time in the manga at all, but she carries herself the way someone completely competent would. In my opinion Lee would have had his ass kicked by Kankurou in the chuunin prelims anyway.
One thousand of Tenten in Part II could take this. That's more than an army's volley of weapons – it's an unimaginable amount of throwing weapons. The weapons flying through the air would blot out the sky, and that's probably with only a few dozen throwing. This is a well-regimented, reasonably skilled army, and Gai doesn't have the key weapon he needs (underground travel). I don't see any gaps in this volley of attacks, and Gai can't just run into it. He'd run around and away from it, but one thousand is too many: their combined stamina and number of weapons is insane. I guess it's fair to say that Gai couldn't die to this force, but I don't think he could beat it.
If it's Part I, I don't think the Tenten can keep the volley up without gaps and Gai could use ura renge to get close and then it would be a slaughterfest.
Five hundred is probably underestimating, too, if we go off the idea that this is anime Tenten. Probably best to use that Tenten as the other one has shown even less. If it is that Tenten, then she can also manipulate the same weapons over and over again.
Gai's kuchiyose powers and other jutsu may be useful, but he would need to show them for me to be convinced. It's not impossible to slaughter this army, but Gai has not used anything appropriate. A small summon never seen in combat is not going to be of much help. ZabuzaDemonGod, I understand the temptation to support Gai due to his skill, speed and power, but what would Gai do in reply to the onslaught?
In case you didn't notice, it's not TenTen Gai is fighting it is 1000 TenTens he is fighting. The amount of weapons, exploding tags and ranged strikes that will hit Gai are almost infinite. Not even he would be able to win, although he may win the fight.