The online game store is known specifically for supporting and selling video games without any DRM (Digital Rights Management). This includes restrictions like requiring an internet connection to validate purchases, requiring a log-in to an online server, and internal DRM software like Denuvo. The idea is that they believe such measures hinder paying customers more than it does pirates, and they want to provide a platform where customers don't have to worry about these things.
As it turns out, Hitman is guilty of exactly that. The popular stealth adventure game by IO Interactive, while mostly available offline, has sections of the game that require an internet connection. This mainly consists of the Elusive Target and developer-made Escalation missions.
As one might imagine, much of the GOG user base was unhappy about the misrepresentation. A GOG representative has issued an apology and explanation.
We're still in dialogue with IO Interactive about this release. Today we have removed HITMAN GOTY from GOG's catalog - we shouldn't have released it in its current form, as you've pointed out.
We'd like to apologize for the confusion and anger generated by this situation. We've let you down and we'd like to thank you for bringing this topic to us - while it was honest to the bone, it shows how passionate you are towards GOG."
That honest to the bone part is a very polite way of saying that some people were being way too angry about this. Some users went as far to leave VERY negative feedback on the game, and even review-bombing its store listing.
As a result of all of this, GOG has removed Hitman: Game of the Year Edition from their storefront. This will be pending until a final decision can be made between GOG and IOI. The store did specify that they do not tolerate review bombing in response to the release, however.