I never had problems with film slipping on my GW670II or GSW690II. When loading a new roll, I'd use light pressure from my left thumb to keep a little drag on the left spool holding the roll film, while winding the start of the film onto the right spool. I'm pretty sure I read a hint somewhere about doing this. Anyway, it seemed to work well.
I did all that and I still got loose rolls, IF i changed brands of film. I never had problems if I kept shooting Fuji or Kodak or whatever. But as soon as I changed a brand, then the next roll would be loose enough for major light leaks. The next roll would be fine.