Danmuji ( yellow pickled radish) is a very important ingredient for gimbap; I never make gimbap without it. It tastes sweet and sour and has a really nice, crispy texture.
Look for it in the refrigerated section of Korean and Japanese grocery stores. It’s sold in airtight plastic packages, either cut into strips or whole, with enough pickling liquid to keep it moist. It keeps in the refrigerator for a few months.