buying GA's on steam that are already in your ibrary

You answered the question yourself, it seems. :3

can I only list the GA , then friend the winner and buy it then?

Yes.

If no other store has steam keys for it (and the game website doesn't) then yes, sadly. And I'd make sure you region lock it if there's a big price/value difference in certain regions, else steam wont let you gift it:(

Check and double check so that your region is not so much cheaper than Europe that it can't be gifted to someone here. Also, check so that it's not one of those games that are actually radically cheaper in your region. Also, keep in mind that someone has 7 days to accept the gift, and if the sale ends and the winner is slow to respond, you might end up having to buy the game at full price. Basically, gifting through steam is a hassle and a half these days.

ugh, it is certainly putting me off the idea.

There's still the possibility to use an alt account for putting gifts in your inventory. But there isn't any workaround for the regional pricing considered while gifting on Steam.

It doesn't matter if you already own it in library or not. Store purchases can no longer be stored in inventory since half a year ago, and it is not going to return.