To my understanding The Offering was the Linux version of Quake and its two mission packs. I might be wrong but that kind of confuses me why GoG would port Quake from a Linux version to Windows. Can anyone explain this to me?

Edit: I seem to be wrong because Amazon has old copies of The Offering that are for Windows.

It's just the Windows version (and DOS version) with that title.

GOG hasn't ported anything, they simply use the title to infer that it includes the mission packs, since the (Linux-only) release titled such was the only release to bundle the add-ons.

Though the subtitle "The Offering" was previously only used for the Linux release, it's now intended to mean "includes the mission packs".

Quake 1... for windows? :O