It's not called 'gunpowder' it's called 'black powder' you can see it on this page:
Cannons
We have reduced the minimum skill requirement for Black Powder to 65, giving it a 70% success rate at GM; we never intended it to be so low in the first place.
Regarding the name of it, it is true that "gunpowder" predates "black powder" by several centuries, though both refer to the same stuff. I went with "black powder" mainly because I just felt nervous about putting the word "gun" into UO. It just didn't feel right. Maybe it should have been called "cannon powder".
Using "Black Powder" instead of "Gunpowder" isn't the first anachronism in UO, at any rate.