From Piotr Szczesniak from GOG.com:
There are a number of distros. We can support just one (which is how Steam is doing it), but since we believe strongly in freedom of choice, that's not our preference. On the other hand, supporting everything in the world is more burden than any business could assume So, the last time we looked into this, we investigated supporting three common ones: Mint, Debian, and Google's Chrome OS. We researched the number of OS updates, how often they occurred, when (and how frequently) various libraries are surpassed and deprecated. We then researched how often, for example, updates to these versions of Linux caused problems with DOSBox, SCUMMVM, and other tools that we make use of for our remastering process.