I was thinking exactly the opposite. This release worked fairly smoothly for GOG, all things considered. I fully expect H2 and H3 to arrive here at some point, probably faster than H1 made it.
Please elaborate. Do you think that, since very few users visit the forum and won't see the negative feedback, the sales were likely strong enough for GOG to bring the other games? The dozens of pages of bad reviews probably won't be considered by most people?
Unlike others, I don't see this as an isolated case, a mistake, but rather as one more step in a fairly clear trajectory that has begun quite a while ago. As such, I think GOG should have been able to expect the reaction, which in my opinion was probably milder than they could have hoped.
The forum reaction was smaller than past debacles (e.g. regional pricing), probably not because people are less invested in this issue, but because there are less active people on the forum than there were in the past, many committed ones having already left GOG for its past transgressions. And the forum really matters little, as it is seen by few.
The game page reviews were, I believe, unique in GOG history, but at the end of the day, it's just 300+ negative reviews, not even a drop in the sea. And the online news articles --- well, CDP has survived much worse (see CP2077).
The image of IOI has been slightly tarnished (e.g. 1.3/5 rating), but most everyone --- justly --- blames GOG (if one is anti-DRM) or the "review bombers" (if one is pro-DRM or pro-new-definition-of-DRM-free).
These kind of bumps are par for the course for serious video gaming companies, so GOG and IOI should be able to patch thing up and find a way to cooperate again. Remember Sony temporarily delisting CP2077 from it PS store? Does anyone believe no other CDP game will be sold by Sony because of this?
I still believe GOG could have something up its sleeve, to perform its two-step-forward-one-step-back dance. It might not be a rabbit (fully DRM-free Hitman) --- it could be a shrew (DRM-free but everything unlocked at game start) or a dove (remove the game and compensate IOI) or a peacock (leave the game as is and compensate IOI).
From GOG's POV, I believe what's more important than Hitman sales is how this has affected other sales (e.g. Blood Omen, etc.). I suspect the effect has been minimal, which is what really matters for GOG's accountants and shareholders.
Now, if GOG could land a day-one pseudo-DRM-free game (i.e. DRM-free more or less like H1 currently is), that could be a great move for them. It probably won't be an IOI game, but these should follow, when the GOG clientele has become more accustomed to these types of releases.