Gold is the premium currency in AdVenture Capitalist. It can be obtained through gameplay during events, by earning certain "Everything unlocks" in Earth or the moon, or doing a hard reset on any planet (first time only per planet). The hard reset will reward the player with 200 gold and can only be earned once.
Gold is not required to progress in the game, but does allow you to purchase big multipliers to significantly speed up progress, via Gold Upgrades. It also sometimes adds small aesthetic changes along with the boosts.
Gold bars cost
Gold Upgrades are special upgrades bought with gold. Like the other upgrades, gold upgrades increase the earnings multiplier. Most of these upgrades are permanent, meaning they persist through normal resets and hard resets.