This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
You fight Gafgarion three times in the game. The strategy to beat him
depends on where you are.
First time: Before entering the battle at the falls, change Gafgarion's
class to something other than Black Knight, and make sure he's
completely unequipped. He'll go down easily.
Second time: To stop his Dark Sword ability, try to use either Steal
Weapon or Weapon Break.
Third time: This is a duel between Ramza and Gafgarion, and there's no
way to win while Gafgarion keeps using his Dark Sword ability. Try to
steal or break his weapon again. If that doesn't work, make sure that
Ramza has the Chakra punch art ability, and keep using it repeatedly
until the melee outside the castle gates clears up. Once you've defeated
the enemies outside of the castle, move Ramza into position next to the
gate switch. Now, rush in and go for the kill.