And you fail to realize Obito was facing naruto and kakashi as well and your not even analyzing the fight properly, Gai lacks ninjutsu which is a major factor here.
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It won't go as smoothly as you make it out to be due to the fact that Gai cannot create such openings as he told naruto do with the use of kage bushins, Might Guy will be facing obito by himself. Even a attempted surprise attack with the use of nun chucks by Might Guy
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. I won't deny if obito's come in with CQC taijutsu against Might Guy, Guy will be able to hold him off with his taijutsu but eventually he will get warped, simple as that.
Kamui warp ends this, as soon as Obito gets a hold of Might guy he get's warped. Simple as that. Not too mention obito can always use human path and snatch Guy's Soul all it requires is a single palm to take his soul away.