Does the version of Quake at GOG contain all official mission packs?

Just to point out, the original mission packs (Scourge Of Armagon and Dissolution of Eternity) are included. Still, Machine Games released two more during the last years (Dimensions of the Past and Dimensions of the machine). You can download the first (Dimensions of the past) from their, eh, twitter account (it's somehow only a link in an old tuit). The second one (and an updated version of Dimensions Of The Past) are not included in GoG's version because they are included in the re-release "Quake Enhanced" which is not currently on this store.

Dimension of the Past was created by a single person (J. F. Gustafsson) who works at MachineGames, and it was released in 2016 although it wasn't official.

Dimension of the Machine involved more people from MachineGames and was released as a part of Quake Ehnanced re-release in 2021. Dimension of the Past was included as well (with slight tweaks and addition of another secret level) and I guess became official as well.

Yes, I read up all about it all the other day, and I am feeling quite excited to get 'Quake Enhanced' ... can't wait.

I've bought or downloaded many Quake mission packs and levels and maps etc over the years. I even got some editors and built a few Quake levels. The only game I have ever done that for.