According to Wikipedia the Doom 3 BFG version was supposed to include the Ultimate Doom version of Doom I.
Everything I'm reading about Doom 3 BFG here on gog just says that it includes Doom I. Nothing indicates whether it's really the Ultimate Doom version or just the original Doom I. The difference is that the original Doom I had 3 episodes while the Ultimate Doom included a 4th episode called "Thy Flesh Consumed".

I don't care whether the game is called Doom, Doom I, or Ultimate Doom in the Doom 3 BFG version, I just want to know if it will have the Thy Flesh Consumed episode or if I'll have to get that elsewhere.

Thanks,
GWGuen

It includes The Ultimate DOOM, which does have that episode.

EDIT: nevermind, I'm not actually sure.

My Steam copy of Doom 3 BFG includes Ultimate Doom with the 4th episode, so the GOG version should include 'Thy Flesh Consumed' as well.

Just to confirm here, yes. The version of the first Doom in Doom 3: BFG edition includes "Thy Flesh Consumed", making it "The Ultimate Doom". However, be aware that it's a special edited version of it packaged for the BFG edition which replaces the red cross iconography on health and berserk pick-ups with that of an ugly pill icon and also changes a few other things as well.

For what it's worth - the Ultimate Doom is the definitive (interpreted loosely) version of the first Doom. Not accounting for the Bethesda store re-releases or John Romero's SIGIL or his map "updates", Ultimate Doom is what id considered the last version of the game and outside of IWAD collectors and general data archivists/hoarders, you are unlikely to find versions of the game that aren't it. GOG's page blurb could probably do better with specifying things but I assume they didn't write the copy and it was posted "as is" after they secured the deal to sell the game here.