As the gears are printed, they could have some tolerances (it depends on each printer). If you see that the connection between gears is very tight and they not mesh well, maybe the module of one of the gears needs to be tunned.
To do this, just open the Art1GearMotor.FCStd file with FreeCAD, navigate to the Involute Gear object and change the module value, then select the Art1GearMotor object, export it to STL and print it. I would recommend you to start reducing it by 0.1mm.
Hope it helps!