Won a GA - but no key ?

you know he has 7 seven days to send you the game?

Wait for him to send the key/gift link. How much time passed since you've won it? And how much since his last session?

you need to read FAQ

How long has it been? Giveaway creators have a week to send the key. Keys don't automatically show up, the creator needs to enter the key or link and then hit a send button. If it's been less than a week just keep waiting. Don't add or message the creator until that time.

The GA creator has 7 days to send you the key ...

View attached image.

for giveaways of less than 50 copies, the creator has to manually "send" the key

You're not even new and you have made giveaways yourself - I don't quite understand your question. You should know by now that giveaways are made by humans, not machines. The creator has 7 days to deliver the gift and the rules clearly state:

If I win a gift, when can I expect to receive it?
Giveaway creators are allowed one week to send the gift after the giveaway has ended. Depending on the type of gift and the giveaway creator, you might receive the gift to the e-mail address you have listed on the account page, a key might be added beside the gift you won, or the giveaway creator might have sent you a friend request on Steam to make the exchange. Please remain patient, and do not contact the giveaway creator during this time. If the gift is not received after one week, you can mark the gift as not received, and attempt to contact the giveaway creator on Steam to follow-up.

The FAQ are an interesting read, give it a try sometime ;)

Unless you've waited over a week already, then it's time to click "Not received" and move on with life I guess and I apologize for my lecturing in this case.

You should sue.

Terorize him and his family. Always works out!

I have won a GA - but where the key should be displayed is " - "

That's because the giveaway creator selected "Gift" while creating it, and probably didn't check back after it ended.
As others have already said, giveaway creators are given up to a week to send the game to the winner, in any way they deem reasonable.

I posted on the wall of OP, but although he is online he doesn't respond to my FR or my post on his wall.

Bugging a giveaway creator to get a key is the most disrespectful thing someone could ever do, unless a week has passed already, which is not the case here. Not all people are equal in the field of social interactions, there're some (including myself) who would be genuinely freaked out by such behaviour. Don't be surprised if you get ignored, and possibly blocked.

And don't assume anything by someone's online status -be it here or on Steam- because they could have left a browser open with an active login, be connected through the app, or just be physically away from their smartphone/tablet/PC/whatever device they're using.

What should I do?

Calm down and wait.
Because someone was kind enough to give you the chance to get a game for free, it doesn't mean that you're entitled to get it as soon as possible.
At this point, I wouldn't be surprised if they wait until the end of the allotted 7 days period to send you the key,.

But it's also highly likely that you'll have received the game when you next come online.
Afterwards, the least you could do is thanking them, and maybe apologize for the trouble. And please, don't do it again, it's quite impolite towards others.