You have to find it in a Gigantamax Den
So in order for your Pokemon to G-max without the DLC, you actually need to find it in a Gigantamax Den, instead of a normal Dynamax Den. However, these are very rare if you don't connect to the Internet, I don't have Internet on my Switch and I've never found a G-max Den. If you do have Internet, then the chances are raised drastically. So, I'm sorry your Drednaw can't G-max, because you need to have found it in a G-max Den.
Hope I helped!