Gunpowder cannot be crafted in Minecraft
Obtain gunpowder by killing creepers
Obtain gunpowder by killing ghasts
Obtain gunpowder by killing witches
Obtain gunpowder from loot chests in dungeons, desert temples, and shipwrecks
Obtain gunpowder by fishing treasure in some versions
Obtain gunpowder by trading with wandering traders in some versions
Use gunpowder to craft TNT, fire charges, and splash potions