Obviously Gai would win ._.
Lee can open 6 gates. Gai can open 7...
Even if they were both reduced to the same number of gates, Gai being able to push his body more than Lee would win.
I'd go as far as saying 5 gate Gai = 6 gate Lee based on the fact that Lee may or may not have Asakujaku (showing less power than Gai with the same number of gates). But its all speculation for now...