Who can tell me where I can buy genuine gaia gold

You cannot buy Gaia gold. It is against Gaia's Terms of Service and your account will be banned if you do.
If you meant Gaia cash, you can buy it with a credit card or many other forms of payment on the site. Gaia cash cards are also available at many stores including Walmart, Target, and 7-eleven.

Gaia cash can be used to buy exclusive items which sell for quite a bit of gold in the marketplace.