One of the trailers specifically mentions such a weapon, no mention of it in the GOG's store page with the Standard/Omnissiah editions comparison. So was this a feature excises from the GOG release, or did they just forget to list it?

Probably couldn't be bothered to list it. Recently GOG have become rather sloppy.

If you mean the Arc Scourge weapon, then yes; I got the Omnissiah Edition and it included it.