I tried adding damage effects to a Gifred toy! Although, all I used were a few Real Touch Markers and a regular utility knife, not even a design knife. However, using a utility knife, which isn't as sharp as a design knife, actually worked out well, resulting in some nice gouges. I focused on the shield and backpack. I weathered it to resemble a battle against several enemy units with machine guns. Incidentally, while this is a candy toy, you often see it in model shops.

Weathering with GFrame Zifred | miragaia's Gunpla works