Here's a link to the CBSA dispute resolution page
It looks like they have flagged your gearbox as a receiver, so you might have to take the angle that you own a clearsoft rifle and the gearbox is simply the internals to your clear rifle.
When I went through this before I made it clear that I use clearsoft in order to comply with canadian law. The downfall of clearsoft is it's plastic and the peices break easier compared to their more realistic metal counterparts, so replacement parts are often necessary to keep my airsoft rifles running. Buying replacement/repair parts is cheaper than buying a whole new airsoft rifle.
Also point out that your gearbox is for a G&G airsoft rifle. G&G is one of the main manufacturers of Canadian approved clear airsoft rifles.
Never lie about what you paid for the parts.
Write a letter, be polite as possible and explain it in detail to them. Most of their concerns with items like these is the demeanor of the importer. If you're the type of person to start yelling and screaming this will lessen your chances compared to somebody who shows reason and responsibility.