In the vast majority of airsoft guns there are 4 main gears; pinion gear, bevel/umbrella gear, spur gear and sector gear.
The pinion (or motor gear) gear acts on the bevel gear, which rotates the spur gear which drives the sector gear pulling the piston back until it releases. If you are looking to upgrade your gears, generally you are looking for one of two things; improved turnover speed (how many times the bevel gear has to turn in order to cycle to gearbox once i.e. Rate of Fire), or improved strength & durability.
Here you will find a selection of gears in different ratios; the lower the ratio the faster they cycle - the higher the ratio the smoother the cycle, albeit slower)