Hobbies And Interests
First Encounter
The first time you fight Gafgarion, in Chapter 13, he is a member of your party. The easy way to win this fight is to remove all of his equipment and change his class to anything other than "Dark Knight" in order to nullify his skills.
Second Encounter
At Chapter 19, Golgorand Execution Site, you will face Gafgarion again with his full complement of Dark Knight abilities. The goal in this fight is to steal his "Blood Sword" before he kills your entire party with his Night Sword attack. Thieves and Ninjas are best for this task because of their large movement range and high rate of "Steal" ability.
Third Encounter
In Chapter 20, Outside Lionel Castle, Gafgarion challenges Ramza to a duel. At this point, Ramza should have the skill "Weapon Break," which is necessary to defeat Gafgarion as a warrior class. This will completely nullify his ability to cause significant damage.
Other Strategies
If you are using Ramza as a Monk-type character, have him learn the "Chakra" art to absorb enemy attacks and make your character stronger. This is especially useful against Gafgarion's "Night Sword."