Galena Ore is an addition from Gregtech. It spawns in the overworld. Its only real use is being processed into additional base materials. When Macerated, Pulverized, or processed in a Rock Crusher, it yields two Galena Dust, with the pulverizer and crusher giving a 50% chance of sulfur dust. In an industrial grinder, it will yield two galena dust and one sulfur dust, and will also give you a silver dust if you use a Mercury Cell instead of water.
Galena Dust can be smelted in an Industrial Blast Furnace for one silver ingot and one lead ingot, or you can put it in an Industrial Electrolyzer for three tiny piles of silver and lead dust plus two tiny piles of sulfur dust.