I am led to believe it was the TRGB amongst other parts (nothing beyond that said) that was found well above the water's edge.
If that information is correct....the presence of the gearbox is useful and in time we shall learn how it most probably got there.
At this time I can think of a couple ways it might have but none have any other evidence to support one over the others based upon any information in the public domain at this time.
What I will take from this (if true) is whatever happened did so in very close proximity to where the gearbox was found.