I am not sure that Corn Head grease is not superior to gearbox grease for a cutter. Not as big of a chance of loosing lube during operation and burning up a gearbox.
Picked up four tubes of corn head grease this morning at a John Deere dealer. Put two tubes of the corn head grease in the leaking gearbox, bush hogged with it for a couple of hours before a thunderstorm hit. Went out and checked the gearbox this evening and it looks good, I don't need to add anything to it. I'll need to put a few more hours on it to be sure but so far so good.