Gai aint the only one who can open gates. Kakashi can open em too. Gai is just too quick to resort to that. We hardly ever see Kakashi open the gates because he is such a skilled ninja that he doesn't need to do it. Gai on the other hand has nothing. If he can't land a punch, he automatically resorts to gate opening. Smooth.
Now, the question becomes:
Who would win? Kakashi (after he opens the 5th gate), or Gai (also after the 5th gate)
I think Kakashi would pretty much just become a speeding entity of Raikiri. Gai would get his ass handed to him on a silver platter.