As a final help I recorded the shim placement used for the gearbox in this tutorial. No gearbox is going to be exactly like this one even among other G&G AEG's but it can serve as a extra helpful reference to give you an idea of spacing or the ratio that shims are added in a given area.

Spur gear- Top:(0.3mm)x3+(0.1mm)=1mm Bottom: (0.3mm)+(.2mm)=0.5mm

Sector gear- Top:(0.3mm)x2+(0.1mm)=0.7mm Bottom: (0.3mm)x2+(0.1mm)=0.7mm

Bevel gear- Top: (0.3mm)x1=(0.3mm) Bottom:(0.3mm)x2=0.6mm

Never try to simple look for an exact placement of shims for any one gun or gear combination. Another person might have the exact gun and gear set that you do, but that does not guarantee that their shim arrangement will work for you. This is true of any AEG.