But Kakashi could use Kamui right? Then he’s not that exhausted. Even a more exhausted Kakashi reacted to Pain’s nail and missile, so regardless, this feat is relevant.
Madara doesn’t make the shield instantly, he just makes it fast, fast enough to match Gai’s attack, neither happens instantly.
Madara doesn’t need his hand to hold it, but clearly hand gestures increase effectiveness of Gudo Dama usage, thus it’s often used. The main point is, Madara’s hand was occupied there, whether it’s need or not doesn’t really matter.
Madara clearly reacted to Night Gai by trying to block it with his staff, just failed due to Night Gai broke it by bending space.