I'd say that my perception of the difference between a GHR and a seated machine leg curl are quite vast, so I would extend that analogy to the exercise I described and a seated machine leg extension.
Moving the distal ends of extremities around the body is (relatively) easy, moving the body around the extremities is (relatively) hard. Compare a decline bench press to a tuck planche. It's not too hard for most anyone to do a BW decline press (and move their arms around their body). It's pretty darn hard for any untrained person to do a tuck planche (and move their body around their arms), which also involves more "body control"--another difference between the machine exercises and the GHR.
If you've done both, you know that there's a lot more going on in a tuck planche than just an upside-down decline BP and a reverse crunch.
That may not be the best analogy, but I hope it gets my point across.
Have you ever had the pleasure of using a GHR? Or done the similar movement on a pad?
I have, and I find them worlds apart from a simple leg curl.