RE: How do you keep a gear on a shaft?
That green fluid was, indeed, a form of glue [= Loctite]. However it appears you had insufficient interference fit for this glue to hold your pinion well enough. There are several varieties of genuine Loctite out there and some hold better than others. You can also use CA [cyanoacrylate = superglue]. Personally if you cannot get the pinion to stay on tight with one of these glues, I'd solder the pinion on. Roughen the shaft, use plenty of flux and put a veneer of solder on the shaft. Using a vice, press the pinion on (carefully aligned) while applying some heat to the pinion. You'll find this holds well, and if you ever need to remove the pinion to change ratios, a bit of heat will melt the solder enough for the pinion to be pulled off with a GWS Std gear puller.
Cheers, Phil