Why does it say that Quake was made in 1998 and Quake 2 was made in 1997? How is that possible? Quake was actually made in 1996.

That is true Quake was released in 1996 but that is not was GOG is selling.
They sell Quake the Offering that got released in 1998.

Oh... that's something different? Sorry.

Well Quake the Offering is like a gold version it just includes the original game and the 2 addons.

If they didn't change it in the meantime you're correct, the gog version contains the 3 cd's with .cue files.
GOG had actuall remove the .ogg music from the release so they put the cd's in.
And in case you want to extract the music from the cd's the user hansschmucker made a handy tool
https://www.gog.com/forum/quake_series/my_oneclick_audioextractor/page1
hope this helps

As I recall, I think that you can just put the CD in a CD player and play it like a regular music CD, but you have to skip the first track because it's just the data for the game, which sounds like white noise (if it will play at all, since it's not a WAV file). Though I'm not sure any of that will help you if you have a downloaded file rather than a CD.