You don't need Two Weapon Fighting to use a Main Gauche. It's a Knife, so everything that cares about Knives works with a Main Gauche. It's mostly a stylistic thing whether you want to use a Main Gauche or a buckler, but, like you said, it's better on the offense. Hook also helps it disarm things, but size differences basically make it so you're better off disarming with a rapier, or even a shield, in most cases.
Huh? Size differences?
"The character with the bigger weapon gains a +4 bonus per size category of difference" - Disarm, page 219.