Steam is changing its game trading policy in order to protect people over the holidays. While you can still gift a game directly purchased from Steam, games cannot be traded until 30 days after purchase. What this is meant to do, is prevent users from buying game keys from third party outlets, only to have those keys revoked when the third party does not make good on their bills.
“We’ve made this change to make trading gifts a better experience for those receiving the gifts,” Valve said. “We’re hoping this lowers the number of people who trade for a game only to have the game revoked later due to issues with the purchaser’s payment method.”
Source: Steam