The SSGSS Goku and Vegeta characters can be unlocked through normal in-game progression, which would involve getting an A rank or higher on specific Arcade mode maps (Hard version of Extreme Gravity Spaceship for SSGSS Vegeta and Hard Hyperbolic Time Chamber for SSGSS Goku) or by accumulating Zeni in-game (Reach a collective total of 300,000 for SSGSS Vegeta and 500,000 for SSGSS Goku).
These characters can also be purchased on the respective storefronts (PSN Store, Xbox Games Store, Steam) as well.