The proof is simple. Gai had whacked Madara's face multiple times with named moves, yet he still failed to break Madara's horn. The only thing we understand is that his horn took multiple hits to break and that its breakage was off-screened.
What's more, Gai's first 4 attacks didn't even make him bleed. Now consider this, Sakura broke Kaguya's horn AND made her bleed with a single strike. While both are already noteworthy, this can be taken further. What happens when you finish punching something? Your fist loses pretty much all its power and momentum. That's what happened when Sakura hit Kaguya's head. Sakura then moved her fist a bit to the side and broke Kaguya's horn with what may as well have been little more than a nudge. Less than even 10% of her punching strength.
Just to put into perspective how strong that horn is, let's assume the horn is at least as hard as Madara's bones. In this panel, Gai directly punches Madara in the spine:
now here madara is standing moments later:
If that were Sakura's punch, Madara wouldn't even be able to feel his legs let alone stand. his spine would be split
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Just to put into perspective how strong that horn is, let's assume the horn is at least as hard as Madara's bones. In this panel, Gai directly punches Madara in the spine:
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now here madara is standing moments later:
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If that were Sakura's punch, Madara wouldn't even be able to feel his legs let alone stand. his spine would be split