Two question first: (1) Does gunpowder count as a kingdom card, or is it a bonus supply pile (like an action version of potion)? It seems rough to have gunpower take up a kingdom slot if not, especially on a board with only one tech. (2) Does gunpowder also supplement other cards? I'm guessing it ties in with other "tech" class cards from your set. But if not, or if you want it to be detachable from your set (it sounds like this is an outtake), you might consider reworking it so that Gunpowder is a oneshot kingdom card that gains you a non-supply bomb when played. Then adjust gunpowder's price, etc., as necessary to make up for the fact that you aren't spending $3 and a buy to gain it.
Also, bomb's below-the-line wording should probably be more like this: When this is the first Gunpower you buy in a turn, +1 Buy (cf. Messenger)