Good luck on that job. There isn't enough motion in a gear box to flush things out. Kero or diesel will loosen the old grease but to get it out I think you will have to disassemble the box?
I have 4 7 tooth columns soaking in Diesel right now. Will be rebuilding them next week. I have not found a better way to remove the old grease and quagulated 600W from them. Most need a good going over anyway. Rod