I'd write them a little letter or call them back.
A reciever for an AR-15 type firearm is the lower part of the assembly, that bears the serial number. ANY other part of the firearm is not considered a firearm and is not subject to control, including the upper reciever. That's for the real-steel, I'd guess the same would apply for airsoft. IMO a gearbox doesn't even closely ressemble an M-16 lower reciever.
I guess that there's a Custom officer somewhere that needed a new gearbox for his gun heh?