This farm uses the fern to gunpowder mechanic in Create Above And Beyond. It's very cheap to make (full material list in gallery). It doesn't use any brass machinery and generates about 1 gunpowder every 6 seconds. This is one of my first create designs so it's likely imperfect. (notably the part where it uses a contraption to pick up ferns) but it should be sufficient for the early game.

You need to manually add a knife to the topmost deployer, add water on top of the stove, make spirit fire by lighting a demon fruit on fire (on top of the netherrack) and set both deployers to right click. Then activating the windmill starts the farm. You might have to use a wrench on the windmill to change it's direction.