There should be a gift copy of the game in your Steam inventory. Select it and the column on the right should state what it contains, and enable buttons to 'View in store', 'Send gift' or 'Add to my game library'. If the game is already in your library, you would just need to 'Send gift' (either to an email address or directly through Steam). If the game is not in your library yet, and the gift states that 2 copies of the game are included, then you would have to redeem the gift first, and send the remaining code to someone else.
If you do not see the gift in your inventory, try logging out of steam, exit the client, reboot your computer, start the Steam client and log back in.
