Rinse the julienned radish in cold water at least twice in order to make sure they aren't too salty. Then, place them in a large bowl and add the remaining ingredients: green onions, garlic, gochugaru, plum syrup, rice vinegar, black pepper (and, if using, vegan fish sauce, sesame oil, and sesame seeds). Also add back the pickling liquid. Mix until the radish is evenly coated. This Korean radish salad can be enjoyed immediately or stored in an airtight container in the refrigerator for several days. Over time, you will find additional liquid coming from the radish.