Gifts that you bought for your fiancée are hers, not yours. Therefore, it is your fiancée that should give Zakah from it when the time comes. For a certain amount of wealth to be subject to Zakah, one year should have passed over its ownership. After one year, you calculate the gold (or money) you have in hand and give 2.5% as either gold or cash. It is not necessary to exchange the gold to cash.