I’d also be interested to know of any knowledge regarding gearboxes for hobby-style outrunner brushless motors.
I’ve combined gearboxes with a few different motors and I’ve had to modify them in some way every single time! One way that has worked for me is using planetary gearboxes where the sun gear is big enough that I can machine the bore bigger so it fits onto the motor shaft. For example I’ve been using NEMA 17-size gearboxes that usually come with a 5mm bore on the sun gear, but I machine the bore out slightly to 6mm to fit the shaft of a 5055 motor and that has worked quite well. If you don’t have access to a lathe to machine the sun gear, I imagine it could also work if you use a gearbox that has a larger bore than the motor and find a right size bushing that you can press-fit into the input gear and accept the motor shaft. Though not with gearboxes specifically, I’ve done this to adapt pulleys to motor shafts and it works nicely.
For a 6374 motor perhaps a NEMA 23 planetary gearbox would be a proportionate size, a search for them on sites like Aliexpress, etc, brings up many similar items.