Gears&Differential that are compatible with brick systems. Load the file into your slicer and right click, select “split→to objects”. Then pick the gears you need out of the stl/step and delete the ones you dont want to use - print laying flat with supports if needed. For most strength print horizontal to ensure horizontal circular layers which are stronger and make cleaner geometry. 

A small nozzle is recommended to strengthen the delicate gear teeth and give a cleaner printing result with better fitment. also check the “Walls printing order” and select Outer/Inter to ensure good fitment.

Filament wise, I advise for PETG, ASA, ABS or CF reinforced ones to ensure longevity.  

Happy printing and building mechanisms :)