Gearboxes are stamped metal of some sort (aluminum or stainless), that doesn't rust. The operator's manual is available online. Go to Landpride.com and select Manuals on the upper tool bar>> Spreaders...
Also, a TBN thread entitled, Re: Cosmo 500, Gear Box Lube?, might be helpful.
After putting in several shots of grease, I finished filling the gear box up to the zerk with 90w Gear oil. As CCM said in the thread above, there is a retention problem using gear oil (the oil leaks a little at the mating surfaces of the gear box housing). But, the leakage is minor so far, and, with gear oil onbroard, I'm confident the gears are getting lubed.
My main concern with the Cosmo is that the agitator spins in contact with the floor of the spreader and wears the paint off in just one load of fertilizer. It seems to me that a thin spacer between the agitator and the floor would help protect the floor. I went to the extreme and had Line-X spray the inside of the spreader with liner, and use a 1/8" thick, high density plastic spacer between the agitator and floor. So far so good. It seems like with these extreme precautions the spreader may hold up for a while.
OkieG