Gold is a resource that you can use to buy/ upgrade buildings and craft Spells. Normally, you can collect Gold from the Gold Mines and from your Captured Gold Lands.

In case you fell short of this resource when upgrading your buildings or crafting some spells, you can always bu some using Gems by tapping the Gold Coin icon at the top right of the screen.

It's not possible to buy Gold directly with real money, although you can always buy some Gems, and then use those gems to buy gold.