Gold & Gems
Like most online CCGs Eternal uses two in-game currencies, one of which you can get through gamepay and th other of whih requires spending real money. In this case the two are gold and gems respectively.
One of the good things about Eternal is that any items that affect gameplay (eg cards) can be obtained (eventually!) without having to spend real money on gems. This makes the game genuinely free to play (ftp).
What Is Eternal Gold?
How To Get Gold
Eternal Gold can be obtained through playing the game.
Every win in ranked mode gives you a chest which contains a small amount of gold, however this is a slow way to get rich. If you're looking to farm gold quickly then you might be better off playing the Gauntlet which is free and can give rewards quickly. If you're confident with draft play then think about the Forge - this costs to enter but can give better chest rewards as well as letting you keep the cards you draft.
What Are Gems?
Gems in Eternal are the paid-for currency. Currently they can only be obtained by spending real money in the in-game store.
All the cards in the game can be obtained by playing it however there are some cosmetic items that can only be bought with gems (currently the Argenport Silverwing Totem). So you don't ever need to spend real money on gems. However doing so will let you build your card collection a lot more quickly than grinding for gold; it will also support the developers.