I dont underrstand. I have received an email : neverwinter nights diamond would be free until today midnight. When I click on the offer 3,79 € are asked..

Check the datestamp and timezone of the email. That expired this morning at 00:00 (CET, I think).

"Gratuit jusqu'au 3 décembre à 23h59 CET."

in french it is written free until the third of december 23h59!???? I still dont understand U still normally have this free for the next 6 hours.

There are quite a lot of free redeem codes wait for you to grab (free as beer, of course):
https://www.gog.com/forum/general/winter_sale_201_clearinghouse_thread

Just another day at GOG HQ... :/

Edit: PM sent.

You can tell someone is new here when they haven't yet realized GOG always makes a few mistakes with everything they do.
But yeah, I'm sure someone will be happy to send a code since most people probably already bought the game years ago.

Reasons why i don't check my email often since i know why i use it for.
I just visit gog everyday.

thank you

Do you know how to redeem it?

If not, simply go here:
https://www.gog.com/redeem

They do seem understaffed the way I see it, so it's not surprising. I'm personally not angry whenever I see something like that pop up by GOG - I would be if it were done by an infinitely larger Steam.