Gifted game is asking for serial key?

It should be in your library. Look for "CD key" or something in the game's library page under "Links".

Usually it gives you a pop up though...

Example

EDIT: Added picture.

Lots of games do that, by the way. They ask for a serial key after installation. Usually there is a popup informing you of they key, or you go to that link and get it. The gifter has nothing to do with that key so it's kinda irrelevant to link the giveaway and their name on the description.
If you added the game to your library they key should be there :)

Chaser doesn't seem to come with a key when bought through Steam, although my copy asked for a key as well. All I had to do was cancel it, then launch it a second time, and I wasn't asked for a key. If that doesn't work, might want to ask Steam support, as it's definitely a problem with the game itself.

Iight, gonna try that later. You have a quite a community here =P.

Many thanks

I had this same issue and guys there is no CD Key entry for this game what happened is it installed incorrectly all you need to do is verify the cashe [right click properties] and it'll re-download what it missed.

Its working! I just pressed cancel and tried again. Man that was confusing.
A big thanks to all of you!