Gearboxes don't go from working to not-working simply by changing hands.
Are you sure you have properly compatible and appropriate parts for the gearbox, like battery specifications, wiring, motor alignment and height adjustment, hop unit, and barrel interface, etc.? There are a fair number of ways to get a perfectly functional gearbox spinning into shit because of improper reassembly.
When you give someone solely one part to work on, you implicitly state that all other parts that are not present you are fluent with and therefore assume responsibility for post-modification. When I'm given just a gearbox to work on, I assume the person knows how to install it back into the gun properly.
Contras A-06: F**k it, we'll do it live.