Can not know the gear base material, welding can not talk about the problem can not answer, and specify a reasonable welding process.
Only that the specific material of the gear metal grades, can be based on the gear base metal, select the electrode (wire) grades
When welding something like gears, you need a rod that gives good penetration, and also has a fairly high density. I would probably use 6011 rods. These are the best for any "out of position" weld. Just be sure it's a close weld. Otherwise you'll get a lot of spatter (which you would then need to grind off).