Is this for each and every classic GOG game, or only select ones?

The URL ingsoc85 posted covers the current list of games supported. Due to the way this is being implemented as a universal solution by GOG however it seems reasonable to speculate that at some point in the future a large number of games if not all of them might end up gaining support. It will probably take time for them to go through all 1900+ games in the catalogue, build lists of all the files the game writes to, and add support for the specific game and test it in house, then make it available in a future update.

1.2 officially launches in April sometime, they will probably provide a larger list of supported games then I imagine, and throughout the year going forward. I wouldn't be surprised to see the whole catalogue supported sometime in the next year or so, but we'll have to wait and see how it works out.

Maybe they can use this info to help games uninstall more cleanly -- including those installed/uninstalled outside of Galaxy. It bugs me when the uninstall program asks me if I want to keep the game's save data, I tell it "No", and it still leaves all the files that the games created in locations outside the install directory. (And if I tell it "Yes", it will always leave the base install folder itself, even if there was no save/config/highscore/whatever data stored there, and there is, therefore, nothing left in the folder. Grrrr.)

Yes, many game's uninstallers do not remove all of the files that should be removed. I think that is probably because whoever maintains the installation file manifests at the game publisher or developer or GOG, fell asleep at the wheel. I find all kinds of file turds laying around. It's too bad there is no infrastructure in place with which people could contribute data about such matters and have someone update things in a timely manner like one might find in an open source project with their bug tracker etc. File a report over files not being owned by the package manager with a list of files -> someone updates the file manifest to take care of it in a future push of the software. Bug closed.

:)