I think Doom and Quake are owned by id.
Here's a two-part interview with a GOG founder about the deal, and other stuff (e.g. him calling Steam "malware"):
http://polygamia.pl/Polygamia/1,95338,7498...sc_wywiadu.html
http://polygamia.pl/Polygamia/1,95338,7498...li_wywiadu.html
(English version on the right-hand side of the page)