AJ Styles Gold can only be unlocked by completing the AJ Styles Million Dollar Tower.
Rather than unlocking things through in-game tasks, you can unlock new players, arenas, and titles by spending VC. VC is an in-game currency that you’ll earn just from playing matches. You also begin the game with 30,000 VC to start you off. Titles are the cheapest, at 300 VC each, each unlockable arena costs 1,000 VC, and the unlockable characters range from 1,000 VC all the way up to 8,000 VC depending on the player.
Note: buying the WWK 2K19 Accelerator DLC also unlocks all of these.