By that I mean an online retailer that is allowed/authorized to sell keys which can be redeemed on your GoG account instead of Steam, origin, etc.

Not counting plenty of digital stores which sells Witcher series (GOG version), at this moment only Fanatical is selling some of the GOG games (SNK titles):

https://www.fanatical.com/en/search?drm=gog&page=1

They were once in the bundle, but it seems that they stayed for individual sale as well. Don't know if we can expect there any discounts in the future, but yes, they're here.

Humble Bundle also very occasionally has gog keys

There's this one www.gog.com that sells gog keys, and is authorized to do so.

This may come as a shock, but you can buy GOG games right here.

The roast dais is already piling in, but I get it to an extent. If GOG let other storefronts sell redemption codes, then it could lead to higher discounts than GOG tends to put in place on their own sales. Or bundles, though I'm not convinced that SNK Fanatical bundle was more than a fluke because Fanatical needed a quick turnaround to save face after the last Killer Bundle couldn't get Steam keys restocked for some of its SNK offerings.

You know, I'm a cheapskate and buy almost all of my games on sales these days, but even I have to say that at this point this way of thinking will lead us to people expecting to be paid for downloading free games. Maybe it's the difference in perspective of people who grew up with big game boxes that used to cost an arm and a leg and people who grew up with Steam sales, but I'd say GOG already has plenty of sales and pretty high discounts.

hmm guys I was just asking a question. I mean other client like steam, ubisoft, origin etc do it and it must be beneficial for them so I was just wondering if GoG does the same

Nearing 40 in a handful (not even that, honestly) of years, so I can't say I agree. I also don't see it as the hyperbolic extreme of getting paid to download a free game (though every time Steam cocks up a price and it goes into the negatives, you'll find at least one person going "so, are they going to pay me?" so maybe we're not far out from such a reality).

Finding a better price shouldn't be looked down upon, in general, and neither should the potential for an expanded (and legitimate) market to find said prices. I say that as a cheapskate but I'd say that if I had cash falling out of my ears as well.

Gamesplanet sells tons of GOG keys but no idea how legit they are.

"Authorized retailer since 2006"

https://de.gamesplanet.com/search?query=gog

thank you. It seems that the games that I search for wasnt available. I assumed all games from GoG, espcially old games are sold on gamesplanet as well.

You don’t say why you would want to? You can buy gifts from here if that’s it. Otherwise keysellers are a minefield of fraud, dodgy tactics, and down value the product eg a key to a game is worthless.
Just buy what you want from here via the store, money goes to the right people, and you be assured it’s all above board.