It sounds like they want you to fix YOUR problem with BG3.
Consumer-facing software is incredibly difficult to handle; everyone's PC is just a little bit different, and you never know what edge case is going to cause problems.
GOG Galaxy (the actual GOG software; BG3 is made by Larian) does a pretty good job of keeping things working. Almost all of the posts in here about people having broken builds are people who use the manual installer. Using the manual installer is fine, but that means you also have to be perfect in your patching or you risk game-breaking problems. Unfortunately, in a new game of this size, that was clearly pushed to release faster than Larian would have liked, frequent patches and hotfixes are an inevitability. Managing that much churn manually is bound to result in human error and/or oversight. Thus your problem.
But it is your problem (and possibly Larian's, since they're making BG3 and are the ones pushing so many patches). If you used Galaxy rather than the offline files, and the install broke, then you'd have at least some valid argument for blaming GOG, but you're not.
Fortunately, it sounds like a reinstall of the game using the latest offline installer (which removes all potential for human error in manual patching) seems to fix things for most people.
When I worked tech support, we had a word for your type of error: PEBKAC. It's frustrating for sure, and I'm sorry you're dealing with that, but don't blame GOG for your situation.