I do not know of any specifics gunpowder has directly with magick, however you could always experiment and incorporate it with various workings and/or methods.
First I would note some of gunpowder's symbolism/significance: volatility, protection, strength, warfare, authority, unpredictability, uniformity, etc. Also being a compound of various chemicals [periodic] , gunpowder would be considered by some an Earth element/ correspondent. Keeping these in mind you could always try utilizing it in:
protections & bindings
charging (talisman) or casting
evoking deities or entities (associated w/ symbolism)
Personally when I try to add an article to my workings that I cannot find parallel to my practices or one which has no immediate "magickal" significance- I usually take a bottom-up method to figure out what I am truly working with. Hope this helped.
Gunpowder usually contains sulphur, charcoal, and saltpeter, at the very least. Saltpeter is known to attract and store energy, much like salt, and the same holds true for the color black, so it can be used as a battery of sorts as well as for banishing.
Sulphur, or brimstone is alchemically analogous to the soul, and the spiritual side of things. Sulphur was seen as the male counterpart of mercury, with the two being considered the parents of all metals.
Sulphur is not just seen as the soul, but also the soul of a metal. It is often compared to the sun itself in this regard, and is of three-fold nature, as white sulphur(seen as the state the great work at its completion in a state of whiteness), red sulphur(seen as the state of the great work in dynamic transformation), and black sulphur(seen as the state of the great work in its beginning where there is black nothingness, thus the chaotic empty slate upon which anything can be built).
The Leviathan cross is often seen as the symbol for sulphur so opening it like a sigil might help. Gunpowder itself is martian in nature, obviously, as its purpose is to destroy, so it can be used in bindings, defensive and curse magics as the posters above me have stated.
