Can I get an activation code to play after playing it? (GOG)
I want to add this game to the steam library
so I'm asking you if I have to buy Darkest Dungeon a second time?

thanks in advance :)

If you want Darkest Dungeons on Steam you will need to buy it on Steam. GOG is its own store - not a Steam key reseller.

Yeaaaaah, don't do that.

There should be a way to add games so it shows up in your Steam client library (but not in your Steam store library). I don't use Steam and don't know myself how to do it, but there have been plenty of threads over the years detailing how it's done.

Try that before spending any money to buy a second copy that's not technically needed.

Guess I don't see where achievements and sync were mentioned. Nor do I see where it's specified that it's the store library versus the customer's local Steam library.

I'm looking out for you guys, trying to keep the customer from thinking that the gOg version is worthless to a Steam user (it's not) when instead it can be launched from the Steam client once setup to do so. Mentioning it seems like a good alternative to telling him or her to spend their money at the competition...

From what I understand, the OP is asking about activation code to add the game to the Steam library, therefore Linko's answer seems accurate.

You may add the game from the Steam navigation menu (Games - > Add a Non-Steam Game to My Library) or from the very same place you can activate new key (Add a Non-Steam Game...)
That is the only way and you will not get any Steam achievements nor downloads.