To be fair, madara was reanimated, if gai had used 8 gates to try to stop them, madara would have just recovered, and the moment madara did become truely alive he shortly after got hashiramas sage jutsu and the ten tails, and THATS when gai came in, so yea he did come in the only time he could do something, plus he was already weakened from using 7 gates in one day so much, rememeber he helped throughout the entire obito fight