No one thinks MS is unbeatable, the word itself doesn't apply to anything in the Naruto world. I must disagree however dispite everything that was said I doubt Gai would stand much of a chance against MS.

Sure he is used to dealing with sharingan and he more than likely would be able to beat Kakashi on any given day but MS no. One Gai is only used to dealing with normal sharingan and even so only on Kakashi's level and as far as sharingan goes Kakashi has never been the standard barer.

I really don't see Gai being able to deal with either Sasuke or Itachi, one because both their sharingans are stronger than Kakashi's. Two because of speed, Gai's major advantage over Kakashi is speed and not only are the Uchiha's sharingan better which would make it easier for them to follow Gai's movement but they are unbelievably fast, much faster than Kakashi could hope to be so again they would be able to counter his movements easier.

A blind ninja being able to defeat Ms is just nonsensical. Not only would a blind ninja not be able to defeat anyone who can use ninjustsu proficiently but they wouldn't stand any chance against a normal sharingan much less MS. Plus you are ignoring MS's most remarkable ability which is the ability to affect the physical world and although they primarily use doujutsu, because its stronger than other genjutsus, it isn't the only type of genjutsu there is.