Not long ago a semi-official (licensed by id/ Bethesda but developed by MachineGames) fifth episode of Quake 1 called 'Dimension of the Past' was released as gift to the fans for Quake's 20th anniversary. It can be downloaded for free from several sources, though it comes pre-packaged with the Steam version of Quake. I was wondering why it's not included with the GOG version.
Would it be possible that it's added to the GOG version to be properly playable like 'Scourge of Armagon' and 'Dissolution of Eternity'?

That's the first time I hear this. Are you 100% sure it's included by default?

http://cdn.bethsoft.com/quake/dopa.rar

Its never released anywhere else. Its still un-official mod - not addon. it just mod by one of the ZeniMax devs (probably, d not trust everything what Net tell you)...

...no it doesn't.

EDIT:
...okay, now it does.

https://cdn.bethsoft.com/quake/dopa.rar

Remember that a remaster is rumored (and QuakeCon is coming this week)

Steam version got a beta patch just now, and i think Bethesda client version too

Don't wait that for gog version, is not coming (The same was happened for Doom 1 & 2 some year's ago)