Stats (per in-game day): ~ 30.
Most perferably, find a large body of water and make a 13x9 platform with it going all the way down to the ground. Or find an island of similar size.
2. Mark the centre lane of the 13 block side with a diffrent block and push your tamed cat on the block.
3. Add 7x6 blocks wide walls surrounding your cat and excluding corners.
4. Break inside and push your cat on one of the 7 block wide walls. The cat will scare creepers away to the other side.
5. Cover the roof with trapdoors except of the line of blocks opposite of where the cat is at or where the creepers will be running away.
6. Cover every blocks surface with slabs to prevent the creepers from spawning on top.
7. Inside the farm, place 6 carpets, 3 on each side in symmetry like in the image below.
8. On the opposite side of the cat, dig 3 blocks deep on the whole lane.
8. And then on one of the corners, dig an additional 3 blocks and then another 2 blocks.
8. Then dig 2 blocks deep and another block behind.
9. Place a chest in the most deepest end of the hole.
10. Connect a hopper to the chest.
11. Place your soul campfire on top of the hopper.
12. On the opposite corner, place water flowing in the direction of the campfire. This is so that the creepers will get scared from the cat and they will run down this side and fall in teh water which will push them in the campfire, killing them and then the gunpowder will be picked up by the hopper and then inserted inside the chest.
12. Place trapdoors above the water to trick the Creepers A.I into thinking it's a solid block. (Most mobs think that opened trapdoors is a solid walkable full block).
13. Dig down and find your way to the chest and place 4 ladders to be able to get up again.
14. Place torches around your farm to refarin creepers from spawning outside your farm and rather inside.
14. Add 2 of your ladders on the side of the farm to be able to climb up on the roof.
14. In one of the middle slabs on your roof, place another slab to make it a full block.
14. On top of the full block, place 121 scaffolding blocks.
14. Make your way to the top of the scaffolding blocks and wait a time of your choice.
14. Make your way to the top of the scaffolding blocks and wait a time of your choice. just make sure you dont get kicked for being AFK. (the longer you wait, the more gunpowder)
15. Once you made your way down, go down to your chest and pick up your gunpowder!