I have no idea how GTG will work with someone who can't do pull-ups already, there's just a huge gap between jumping pull-ups and pull-ups. I could see the negatives working out, but again, even a negative can be difficult.
I'm working with a guy right now who's ~295, and he's on the verge of a pull-up, though it's been a pretty long time getting there. I used the Iron Woody bands, used a rep scheme like:
3x3, 4x3, 5x3,
3x4, 4x4, 5x4,
3x5, 4x5, 5x5.
With each band, starting actually with multiple bands. So, we'd spend a few weeks on each band. He's now on the mini. He started on a combination of the Large and medium, which I feel ended up being 150-200# of assistance at the bottom.
Took awhile, but it worked.