Depends on whether its 'gunpowder' or gunpowder. 'gunpowder' being the smokeless propelent commonly used these days, mainly composed of nitrocellulose, nitroguanidine etc. Or the real stuff made of potassium nitrate, sulfur and charcoal. Only the latter will be good for your lawn.
But really, just keep it for burning. Especially if its the old real stuff, the smokeless only burns relativly slow in the open.