I have never heard of Quarantine and Google has never heard of it either, so I cannot see how it is a classic, and despite probably getting flamed, some of the titles mentioned are never going to be on G.O.G. because while being good games in many cases, they were never in the classic class!
Googling "Quarantine PC game", gets me 210,000 hits, so, not quite sure whats wrong with your Google. A quick trip to mobygames, would tell you, just like in my original link.
And GOG stands for Good Old Games, NOT Classic Old Games, quite a difference. There are already a few games on sale that weren't regarded as classics, simply because they were overlooked, underrated and long forgotten back in their time.