What did gun powder replace?

Answer 1

A long and grand history of mechanical force based projectile weapons, such as the crossbow.

however if you mean modern gun powder, then it replaced what's know as "Black Powder" (quotes because crude homemade black powder is actually more on the gray side).

The black powder and bullet would be packed in the barrel of the gun, and as late as the American civil war an explosive percussion cap was used to spark the powder, the powders combustion would create expanding gases and the metal ball would either hit its target or miss. Gun powder today is mostly man made compounds, such as cordite ( a smokeless propellant first developed in the late 1880's). if you want the nit picky details (such as grains, load weights, etc...) i suggest looking up various ammo manufacturers