Yeah, the TM's have that easily removed gearbox. The CA's however are just stupidly hard. No amount of banging on a book will dislodge the gearbox: If you physically look at how the gearbox goes in, and push the gearbox to the rearmost it can go, it physically cannot be dislodged from the gun just because the gearbox is a bit thicker, and the interior shell is smaller than the TM. It has to be wiggled out, using the chamber doors and the mag topplate.
But, this being said, the CA pretty much needs no gearbox upgrades right out of the box. So I wouldn't bother removing the unit. The hopup unit on the other hand, is a definite must have, and doesn't require the gearbox to be taken down.