I think Tsunade takes this, due to Body Pathway Derangement. Gai has to make contact with her to do anything. Also Tsunade likely has quite a bit more to show off, though that's not really relevant until it's shown in the manga.
Though if Gai goes 7th gate, he has a serious chance and may prove too quick for Tsunade to hit him with Body Pathway Derangement, so in that case, it could be argued that this might go either way.
Some food for thought: Imagine how OP Tsunade would be if she used 8 gates.