The Talos Principle: Road to Gehenna DLC Steam Gift

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"Please note, that due to recent Valve Steam Gift policy update Steam Gifts for games that are covered by VAC ban program can no longer be added to Inventory. Make sure that you are logged in to the proper account before entering the link as Steam Gifts can be only added to your library or rejected.

How do I use a Steam gift given to me?

If you do not already have Steam, download and install Steam here. Once you have Steam installed, you will be prompted to create an account or login to an existing Steam account. If you do not have an account, please create one.
You will receive an e-mail containing a link that will open Steam allowing you to redeem your gift. After clicking the link, you will receive a notification at the top right corner in the Steam client informing that you have a new gift.