The only way to use grease in a gearbox, and I upkeep several gearboxes.
You have to mix the oil and grease together in a bowl or separate container. It makes a thick sludge. Then pour it in the gearbox. I use a suction gun for some applications. If you don't pre mix the two together, it will never mix. If you pre mix, it will never separate. It will stay in a bad seal mixed that way. Case in point. My dad was using a hydra push manure spreader for clean out. Probably 800 seven cu yard loads. We put it in a shed. We don't spread with it. We keep a 4x4 tractor by the shed to pile the manure high and make more room for tractor and spreader. Piece of string got on shaft and got the seal. Adding gear oil every other trip. I put mix in it, he checked a couple times, it was full, he kept cleaning out. We did fix it later.