Updated design of my earlier gunpowder farm. Notable changes are a different method of picking up ferns from the fire and removing unnecessary sail blocks. It's cheaper in general, takes 6x less sail blocks and is about 3x as fast.

List of building materials in the image gallery.

After building using the schematicannon. You need to manually ignite the demon's dream fruit on the netherrack block, add water on top of the stove, right click the deployers with wrenches to get them out of "punch blocks" mode, give a knife to the topmost deployer and lock the topmost drawer as well as adding some slimy ferns into it. Right click the windmill bearing at the top of the build to start the farm. Doesn't require any further upkeep and will passively generate gunpowder at a rate of about 1 per 2 seconds.