In order to activate your code please follow the instruction below:

Log in to Guild Wars 2 with the account where you want to unlock the item.
Open the Black Lion Trading Company panel. The default hotkey is [O]. You can also click the lion icon in the upper left corner of the game screen.
Click the "Redeem Code" link in the upper left corner of the Store homepage.
Click the "Redeem Item Code" button.
Enter or copy and paste your gem code into the field that reads: "Enter your code"
Click Redeem to receive your item!