My goldfish have never had a gash that big but they've definitely gotten their fair share of scrapes and bumps off the decor in the tank. I even had a young oranda orange cap scrape up his eye and he spent a few weeks with a puffy/cloudy eye that I was worried about. But they always healed, it just took time. I threw a little salt in the tank and let the fish do the rest. My plan was always to quarantine and do more with medication IF it seemed like it was getting worse, but so long as it appeared to stay the same (e.g. it was healing slowly and I couldn't tell day-to-day), I didn't want to go overboard and do something to make it worse by accident. That wound might warrant special treatment upfront if others with more experience suggest it, but my advice is to just leave it alone and monitor it, and only take action if it appears to get worse.