This is a simple ball drop machine. It is designed to be easy to print and very tolerant of printing flaws.
It prints in 4 pieces. The gears are attached with metal pegs and a small washer.
Everything was designed in openSCAD except for the upright portion of the frame which was done in tinkercad.
The pictures are ABS printed with 0.2mm layers. Two 3/32" pins hold the gears in place along with #6 washers to make them spin freely. The marbles are 3/8" ball bearings.