Yet another example of hardware that's not listed in the parts books. In this case, the Sprint GT. And the 1750 parts book shows an M5 nut. But it's a machine screw!
If your GT door handle stalk has an internal thread, I think @divotandtralee may have nailed it with an M5 x 0.8 x I'll guess about 20mm long with both a flat and split lockwashers.. Head would be a phillips panhead. Could be an M4 though; just don't remember.