Giveaway

Either by gifting it to them via steam or simply entering in a key are the main ways of doing so. It depends on what you have.

you send gift via mail or steam
or just add key or send key via mail. =)

If it's a key, you can put it in here. If it's a gift, you can send it to the winner's email which will appear in the link above as well.

On the page of the giveaway click on "Winner".
You will have an option to add a key. If you put a key there, you also have to press the "Sent" button or the winner willnot be able to see it.
You will also see the mail adress of the winner on that page so if it's a Steam gift you can send it there.
You can also choose to add the winner on Steam if you prefer.

They never accept my friends request on steam to send gift.