The easiest way to unlock SSGSS Goku is by simply Pre-Ordering Dragon Ball FighterZ. Not only will you unlock him for free, but you will be able to use him immediately after beginning your game. However, should you not have had the chance to do so, you can also unlock SSGSS Goku during Gameplay. In order to do so, you will have to beat the Arcade Ladder, the Hyperbolic Time Chamber Course to be exact, on Hard Mode with at least an A Ranking. Alternatively, you can also purchase SSGSS Goku for 500k Zeni in the Game's Store.