Unlike some Gowlandflexes, yours has matched viewing and taking lenses. That doesn't mean the actual focal lengths are identical - there can be sample variation - but it means that a large focal length mismatch is unlikely. If they are the same or very close to it, then the mismatch will persist throughout the focusing range.

I think the likeliest explanation is that the taking and viewing lenses are not properly adjusted to match flange focal distances. Since you're reporting that the on-film image is back-focused relative to what you see on the GG, it appears that the taking lens needs a bit more extension. Certainly I would start with the viewfinder vs rear GG test that Tin Can proposes, and see what you find. But if the error that you reported from your initial film tests is confirmed, I would try adjusting the mounting of the taking lens before messing with the mirror.