Iron Golems Won't Naturally Spawn In Custom Biome

How do you get Iron Golems to naturally spawn in your custom biome, I technically have done this before but completely forgot how to do it, and I don't have a backup of that file anymore or any screenshots to help me. I tried setting Iron Golem as a creature or a monster, but it refuses to spawn. Also the top layer is gravel in this biome, and it refuses to spawn.