expopower: The broad-sweeping nature of the statement makes its accuracy questionable, especially after taking into account the variety of established storefronts through which GOG keys can be purchased. Surely they could not all be illegally reselling GOG games...?
There is a big difference between selling something illegally, and selling it legally but unauthorized. What has happened in the past is that people were buying massive copies of The Witcher 3 for example through Russia to get the cheapest pricing, then reselling it elsewhere online. That is not necessarily illegal, but it is not authorized by GOG and the stores that sell those copies of the game are not officially authorized by GOG to do so and do not have any formal business partnership. That is what "unauthorized" means. It is dirty unethical business practices, but depending on the exact legal agreements when purchasing the games in the first place from GOG and the laws of the countries involved it may not be illegal.
People confuse "not illegal" with meaning "authorized" or "legitimate". What I'm claiming is that GOG executives have stated clearly before that the only place to buy authorized copies of GOG games is directly on GOG.com, therefore any copies of GOG games available elsewhere are unauthorized by definition. So unless GOG has established a business partnership with Humble or any other stores that are selling GOG keys, then those sites are selling unauthorized games that were obtained in a shady manner. This is not unlike sites like G2A.com for example that sell Steam keys obtained in similar shady fashion, where at a later date sometimes Valve or Origin or whatever publisher decides to invalidate all of the games and remove them from peoples accounts due to being obtained through means that violate their terms of service agreements. Again, this does not mean that any laws were broken nor that any criminal activity has occurred, it means only that terms of service agreements may have been broken and such companies can take action to remove the games and/or invalidate them, or to terminate customer's accounts who have games that violate the terms of service.
To the best of my knowledge GOG has not removed any games from people's accounts nor terminated accounts to date for people who have acquired games this way through shady unauthorized means, and they haven't indicated anything suggesting that they will in the future either, but depending on the fine wording of their EULAs and other agreements it is possible that they could if they wanted to. They probably wouldn't because doing so often comes with undesired customer backlash.
Thus there may or may not be any consequences to customers buying GOG games on 3rd party stores selling their games unethically. Nonetheless, unauthorized is unauthorized whether or not it is illegal, and that does matter to some customers. I personally wont knowingly buy a game from a store if I know that the store is obtaining their games through unethical means such as through 3rd party accounts buying games through Russia to get preferred prices. I want GOG to succeed and grow because they're doing a wonderful thing for PC gaming and I want to support them in a way that benefits GOG, not take advantage of them in a way that benefits only me and some 3rd party ripoff site.
I'd strongly caution people against doing so with Steam games though because there have been cases where customers ended up losing their games afterward with Steam/Origin crackdowns and that would really suck to buy a game and think you saved $5 or whatever, then end up losing the game and having no way to get your money back afterward.
Having said that, I wont criticize or judge anyone who chooses to buy such unauthorized games either, to each their own. I just think people should be aware of the truth of the matter when they decide to make such purchases, that they are aware of these matters and that they're making a conscious decision either way rather than potentially end up getting ripped off and saying "Well I didn't know that!" afterward. I've seen that too many times in the past and it's sad to see people upset after having games removed from their accounts on any gaming service.
expopower: For an official source, I don't have a direct repudiation of this claim, but refer to the list of suggested retailers through which
The Witcher 3 may be purchased. It becomes clear that the tweet made by GOG was not entirely correct - since I don't believe they would endorse other sellers (such as uPlay, which sells GOG game keys rather than GOG games distributed through its own platform), only to throw them under the bus by accusing them of illegitimacy.
GOG.com and CD Projekt RED are two separate subsidiaries of CD Projekt. What stores are authorized to sell GOG.com video games and what stores are authorized to sell CD Projekt RED's Witcher 3 game are two completely different things.
The Witcher 3 game is authorized to be sold by the stores listed on the website you link to, and that may vary around the world. Those are agreements that CD Projekt RED has with the given stores. I'm not completely certain what distribution platform you get a Witcher 3 key for when you buy the game from any of the listed authorized stores, but one thing is certain in that case, which is that whatever you get, it has been authorized so it doesn't really matter.
The statement about what stores GOG authorizes to sell GOG games, pertains not to CD Projekt RED, but to the games available in the GOG game catalogue being sold as products that redeem on GOG.com. If GOG has not authorized Humble store to sell the GOG version of any GOG game, perhaps in some agreement between GOG, the publisher and the other storefront, then that store is just not authorized officially to sell the game. It doesn't get much simpler than that.