I suppose you DO know that the GZ is supposed to idle at 1200 - 1400 rev/min? The "screw" you "found" (if it's the one with the big white knob) is not meant to be used as a starting aid, but used to set the idle when the engine is HOT.
Right - get the engine running by whatever means you can, then let it get really HOT (go for a ΒΌ hour ride, not just let it get hot while it stands)
Get the choke off as soon as you can, & if it stalls, screw up the white knob a little, until it's running. When it's nice & hot (& ONLY when it's nice & hot!) adjust the white knob until the bike sounds as though it's doing 1200 to 1400 revs. This is your starting point. On subsequent days, when the bike is HOT, you can tweak the knob SLIGHTLY until you get it running to your liking.
A lot of idle problems seem to be caused because people think that a bike idle should be as slow as a car.
:2tup: