Open the purchase from My orders and read the delivery information and messages. Different products use different delivery methods; there is no single download or inventory location for every G2U purchase.
For in-game delivery, follow the product and order instructions and check the correct game, character and account. For a service, read the progress and completion messages from G2U.
If the order is marked delivered but you cannot locate the purchase, message G2U in the order chat with the order code and what you have checked. Do not confirm receipt before checking the delivery.