Yes, well there is some context around the utterance that might help. In the scenario, Gurdjeiff was a teacher, and this was advice to his pupils. In that sense, he was referring even to information offered by he himself to them, as in; don’t take my word for it. Also, what he was teaching them was a belief system in which there is no distinction between scientific and unscientific. That is, even that which we’d regard as spiritual or emotional matters are the result of measurable processes and conditions that can be studied and subsequently controlled via an understanding of their working.
While I would agree that our perspective of the metaphysical requires a certain flavor of faith, or at least conclusions based on projection of known upon unknown, Gurdjieff would have asserted it’s only that way in our view, whereas objectively—above and outside of our common sphere of conventional consciousness—that which we consider metaphysical is just as plain, measurable, and verifiable as what (we think) we know to be physical.