Tamiya Gearbox set
Includes frame, gears and small parts for assembly of a complete gearbox
Tight tolerances, might need to drill out holes to 3mm and it is ideal if you use a metal 3mm hex rod instead of the 3d printed ones.
The 'gap' file isn't necessary and may cause tolerance issues that could be avoided if not used.
Model based from the Tamiya 70168 Dual Gearbox
You will need to supply minor parts for the build, e.g. screws.
Parts can be used to assemble other types of gearboxes from Tamiya such as single motor configurations however you will need a different frame to support that.
One single motor config would be this one by NickJallday: https://www.thingiverse.com/thing:1102982