Gunpowder was an explosive substance that was used, mostly in weapons such as cannons, to explosively propel objects. It was used on Earth and on some primitive planets such as Ribos. (TV: The Ribos Operation)
In Venice, in the year 1580, Guido detonated a store of gunpowder to destroy a group of Saturnyns. (TV: The Vampires of Venice)
One of the most notable uses of gunpowder was in 1605, where Guy Fawkes and his anti-parliament plotters tried to blow up the Houses of Parliament withg several barrels of gunpowder beneath it. (PROSE: The Plotters, GAME: The Gunpowder Plot) The Eleventh Doctor used a few of these barrels to launch a Rutan ship trapped beneath Parliament when he temporarily placed Parliament in Earth orbit. Around the same time, Rory Williams used a cartridge of gunpowder to open a locked door leading to one of the ship's missing power rods. (GAME: The Gunpowder Plot)
While searching through a manor near the Terileptil escape pod, the Fifth Doctor and his companions discovered gunpowder, power packs and the mark of a high energy weapon. The gunpowder was residue from a fight between the owners of the manor and Terileptils. (TV: The Visitation)
In order to fight Windigos, the Eighth Doctor created flash-charges. These were made by adding iron fillings to gunpowder. The light created upon the gunpowder's detonation deterred the Windigos. (COMIC: Bad Blood)