Green’s Dictionary of Slang

(US) a brand of dynamite.

1871		W.P. Blake Notices of Mining Machinery [...] in Use Chiefly in the Pacific 14: For the new ‘giant powder’ a three-quarter inch drill is used. [Ibid.] 20: Gunpowder, fulminate, nitro-glycerine, giant powder, or other violent explosive.
1887		Courier-Journal (Louisville, KY) 26 Jan. 1/1: Two hundred cases of giant powder exploded on a freight train yesterday.