During the battle after he sends Pippin back up to the Citadel, three lard rams and trolls push the battering ram known as Grond toward the gate of Minas Tirith and Orcs are shouting Grond, there is a moment where Gandalf has a look of shear terror and panic on his face as if he'd seen it before.
Why is he afraid of it?
From the LOTR wikia entry on Grond:
Grond was an immense battering ram said to be 60 feet high, 150 feet long, and as thick around as an enormous tree. Fire burned in its great maw. It was operated by a team of great beasts and several trolls were needed to utilize it. The ram contained so much innate evil, that it is said the creatures pulling it would occasionally go mad and run through the battlefield, trampling anything and anyone underfoot. Aided by the "spells of ruin" laid on it during its forging in Mordor, and by the Witch-king of Angmar during the Siege, Grond smashed open the formidable gate of Minas Tirith with just three blows.
Gandalf was terrified because he knew the gates would be breached as soon as Grond reached them.
The only answer implied by Tolkien is that Gandalf sensed, when the gates were breached, that he would face an old foe of terrifying power -- one that he believed could be his match. He would face the Lord of the Nazgul.
