The barrier is tall enough to prevent Gai from getting out before he gets nuked anyway.
Madara was able to wrap himself in the Black Chakra before Gai reached him. Sure, Obito will getting hit around some during this match; I know that much. But this is how it will end as Gai isn't going to pressure him enough for the whole match to prevent him from using Bijuu Dama. In fact, there is chance Gai might not even get a good hit on Obito.
We have already seen that the Black Chakra can
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Evening Elephant. The issue is that Madara didn't cover himself in it from all directions, he only made a frontal shield, so he ended up getting hit. All Obito needs to do at that point is wrap himself in it and then Gai won't be able to get past it with the air pressure of Evening Elephant..and I'm pretty sure that
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.