Gold is the premium in-game currency which can only be obtained with real world currency. It comes in packs of various quantities which can be purchased on the Official Nether Website (requires login).
Gold can be used to purchase premium cosmetic items such as some helmets, wrist armour from the Marketplace and the Prepper and Soldier character from the Character customisation menu upon login or revival.
Additionally, bonus Gold can be obtained with the "Believer" or "Chosen" game packages. Not only does this support the developers but it also rewards the players with Gold bonuses at a much discounted price as opposed to buying Gold straight from the website.