Buying Gem Codes off Amazon is completely fine as long as you make sure that you're buying from the official NCSoft account, not someone acting as an independent reseller.

The only official GW2 Gem Card listing on Amazon US is this one: Guild Wars 2, Gems - 2000 - [Online Game Code] by NCSoft. They've been out of stock since the Gem Card sale a month or two ago, but will be back in stock once ANet gives Amazon more codes.