The Black Lion Trading Company is open for business, and business is available at a retail store near you. Before gems could only be purchased with real life money in game via credit cards or PayPal. Now an official Guild Wars 2 box withholding a serial key to unlock gems is available at select retailers across the globe. Once redeemed gems cannot be traded to other players or sold; however, gifting gems will be available in the near future.
Not in stock at your retailer? Just ask to see when their shipment of Guild Wars 2 gem cards is coming in. Further information regarding the use of the serial card can be found (here).