You can trade resources for gold in the market window. In order to be able to use Gold Trading, you must have obtained at least one additional city and you must have built the marketplace. For security reasons, you can only send/receive 200 gold in one trade. This feature is live on all worlds released after February 10th, 2015.
Some payment methods have a verification period, during which you cannot trade your Gold. It may take up to 42 days (Direct Debit) or 60 days (Credit Card, Paypal) to verify a purchase.
You cannot trade free gold gained from daily logins, email validation or other features.
If you want your gold to be available for trading immediately, you can use a different payment method like Bank Transfer, Amazon Pay, Sofort, Paysafecard or Giropay, if these are available in your country.