I'm no expert but I think GT85 is "thinner" than normal chain oil, so it is useless for lubricating parts which are under high-stress (i.e. drivetrain), but for gear-cables it's great.
For the drivetrain I think you need something more viscous, and if you're already using chain oil, adding GT85 thins the oil out too much and you may as well have not bothered oiling your chain at all.
Now I read back I'm not sure how clear that is... But I read something similar someplace.