Neat, so there's going to be one integration which is done properly.
They use a really shady method for Steam, Battle.net, Origin etc. There's this FriendsOfGalaxy repo on Github which provides most of these "third party" plugins. The way they work is that they let you login with a limited functionality web browser (for instance there's no address bar so you could verify it's the legit website) and then snatch your session cookie.
It's a method often used in malware and they use it to circumvent ToS - using official APIs would mean millions of requests, which would lead to the key getting revoked unless they negotiate a special deal. In addition, the API is more limited in scope.
So by using a "third party" repository, they're absolving themselves of any responsibility.
The session cookie essentially lets you perform anything, from reading all chatlogs to doing purchases in store - though the latter occasionally asks for CCV.
Sure, the code is open source and in the repo, but there's no easy way of verifying that your client is running that same code - not to mention a rogue update could lead to users' session cookies getting transmitted to a third party. This doesn't even require malice from FriendsOfGalaxy, one of them getting phished is enough.
But since people want an unified client and also hold GOG in high regard, nobody really gives a toss. Nor do people really understand the implications of this.