Your purchased Doom copy of course ; ) (just search for it, come on, the top results give it to you - just like the search results for "how to run myhouse.wad")
GOG's installers are made with InnoSetup, you can use a tool like InnoExtract to get the files out of them (source at https://constexpr.org/innoextract/ or in MacPorts/Homebrew).