I saw the recent bundling of the Doom series of games from ID Software and was really looking forward to playing them again when I noticed that it was only available for Windows. Any chance that you might have available for Mac users at some point..I know some titles have used DosBox to emulate them.
It's probably best to ask GOG that question via their support forms. Doom here on GOG does come with DOSBox so I would have thought it possible for them to do in technical terms, and once you get the WAD files you can use any of the numerous source ports, which I imagine exist on Mac too.
However, this is Bethesda so it might be that GOG only have the rights to distribute the Windows version. If that is the case then if you really want to buy it from GOG you will be forced to extract the files from the InnoSetup installer somehow in order to get the WAD files which you can then make use of in your own DOSBox or source port of choice.