shmerl: If GOG wanted to do that, they could. Not for sales, but for downloads. They can count downloads per OS, and have a deal with Aspyr to pay only for Linux ones for example. I don't see any technical issue with it. But may be there are other tings involved (privacy regulations or what not). But I'm not sure why for example it would work for Valve, and not for GOG.
For all we know, Valve counts it according to the platform of purchase, or the one with most minutes played after 7 days.
I agree GOG could do it according to downloads, but what if I'm a compulsive completionist and I download all the installers: Windows, Mac, and Linux? Will my purchase count as 1/3 Windows, 1/3 Mac, and 1/3 Linux? Would I have to download it twice for Linux in order to make sure it counts as a Linux sale?
It's sad, but DRM/client-dependence comes in handy for things like these.