If the gift card was purchased using an external method of payment (eg. credit/debit card) steam has to wait for the payment process to totally clear before releasing the funds from the gift card. Why....it's a safe guard in case the method of payment was stolen and they need to revoke the gift card and anything you brought with the funds from the gift card.
The same would count if the gift card was brought with wallet funds that had been recently added via external payment method.

Originally posted by Rando:

Stupid gift card wait

So I received a gift card from my friend and it put me on a wait. Yes I know that it's like "your account isn't verified" but both of our accounts are verified. PLUS THIS ISN'T EVEN THE FIRST TIME I HAVE RECEIVED A GIFT CARD FROM THEM! I need someone to explain this to me like I'm a 4th grader plz thx, or get steam to lift the timer

If the purchaser didn't verify their payment method on the Market before they sent the digital gift card through Steam, you must wait the 3 days for it to be verified to use on the Market.

This is nothing new. The Market has had this requirement since the beginning.