Is the gearbox empty?
When I tried to personally bring a complete gearbox (no motor) across the border, the CBSA agent asked me if it was a core part of the airsoft gun... I answered yes. I guess it's true that, even though, on its own it does not function, it does play a major role in determining the fps of an airsoft gun.
Now, before everyone gets all mad at me... Yes, I know the same thing can be said about just about every other airsoft part (barrel, bucking, body, etc), but you can understand how any doubt will be dealt with on the side of caution. Worse yet, even IF the gearbox has gears, piston, springs, cylinder, etc., they would have no real practical way of testing it.
If what they've held are the shells... well, then I'm just as confused as you are.