Is the (magic/non-magic) bonus of a main gauche added to DB if not used as a weapon?
Depends on what you as a GM consider that bonus to represent.
A weapon is, say, "+20". In the combat mechanics, what that means is very straightforward, but in terms of simulation and roleplay, what does that mean?
Is the +20 for superior materials, that tends to cleave through armor like cheese? Well then no, it probably doesn't add to parries, because the property giving the bonus doesn't affect how nimble it is. You might get an increased chance for your attacker's weapon to break if your parry is successful, though.
Is it for superior workmanship? Is the weapon so ergonomically perfect that it's "distilled joy in the hand", it moves like you were born with it? Then yeah, its advantages probably apply to active defense as well.
Same thing for magic bonuses, you as GM have to decide
why the bonus is there, what it represents, if you want your answer to be consistent with
both the game mechanics
and the story logic inherent in your setting and scenario. Is an item of "+20 defense" something that warns you, or moves you out of the way, or interposes a force field, or does telekinesis on incoming objects, or what? Does the +20 apply only against solids? How about acid spit? How about aerosols, or smoke, which is usually particulate solids? If you had been downwind when Mt. St. Helens erupted, how much of that +20 would have applied against the ashfall?