gift_cards_controller.rb

 Spree::GiftCard.transaction do
          @gift_card = GiftCard.new(gift_card_params)
          @gift_card.save!
          # Create line item
          line_item = LineItem.new(quantity: 1)
          line_item.gift_card = @gift_card
          line_item.variant = @gift_card.variant
          line_item.price = @gift_card.variant.price
          # Add to order
          order = current_order(create_order_if_necessary: true)
          order.line_items << line_item
          line_item.order=order
          order.update_totals
          order.save!
          # Save gift card
          @gift_card.line_item = line_item
          @gift_card.save!
        end