Considering Kabuto is taking on Itachi and Sasuke with relative ease (thus far, at least, he hasn't even sweat), I think he could take Kakashi and Gai no problem. It sounds like Izanagi will be Itachi and Sasuke's luck card. Kakashi and Gai have nothing even remotely close to this.
Itachi is better at ninjutsu than Kakashi. Sasuke is at least on par with Kakashi, if not a little below. Kabuto is having little to no trouble with them so far so I don't see how Kakashi would pose a threat. I'm generalizing here since obviously they have different techniques, but I don't see the point in going into specifics in this case.
Kakashi
- Kabuto was at least Kakashi's level in Part 1 for christ sakes
- Add Orochimaru's DNA on top of that and he already passes Kakashi (He steamrolled Yamato)
- Add Sage Mode on top of that - Kakashi is nothing to Kabuto.
Gai
- If Dragon SM is anything like Toad SM, then Gai's taijutsu might not be that effective against Kabuto's natural energy enhanced body.
- Plus, he has the Hozuki Clan's Hydrafication which renders nearly, if not all, physical attacks useless.
- Even if he was damaged, need I remind everyone that Kabuto is an expert medical ninja with sense surpassing that of Tsunade in her prime.
None of Kakashi and Gai's attacks could even hit Kabuto to be honest. Kabuto has insane reflexes due to his ability to sense the natural energy and environment around him. He was even able to
physically dodge Sasuke's Susanno arrows - something even Kakashi couldn't do. Aside from people with obvious speed enhancements (A, Naruto, etc), I don't see anyone else being able to do this.
If Gai and Kakashi want even a sliver of a chance, then Gai will have to take off his weights and jump into the 7th Gate while Kakashi activates his MS and has Kamui ready to fire. But even then I'd highly favor Kabuto to win.
Dragon SM gives Kabuto such a huge boost in power, speed, reflexes, techniques, etc to the point where it's probably lucky for the alliance that he ended up facing Itachi and Sasuke. Seriously, there is no one else available right now who can match him.