I received a gift card. The instructions which accompany the gift card include the following:
To activate, sign the back of the card. In some cases, activation may take up to four hours.
What, you think I write that slowly?
Bonus chatter: Yes, I know that the reason is that it may take up to four hours for the purchase to propagate through the system, and if you sign the card immediately after purchasing it, you may still need to wait up to four hours. That didn't apply in my case because I received the gift card in the mail.