I think that given that the metalwork and steel quality weren't highly enough developed to produce anything close to a gun, or even a cannon, the Romans would have primarily been making bombs with gunpowder. Loading big gunpowder-filled stones (hollowed out) or maybe amphorae into their ballistas and letting fly, or placing under city walls while sapping. Would work wonders. Romans were great siege engineers.
Great thinking! There I go thinking about military technology straightaway Yes, absolutely, lots of passes in the Alps just waiting to get blown up to make road building easy. Should vastly improve the economic worth of some regions and bolster trade. Perhaps, since travel to northern Gaul and farther Spain will be easier now, they will at least have a fighting chance at keeping up economically with the eastern provinces?Romans being great engineers, might their main interest in explosive powder not lie in more peaceful uses? I'm thinking mining and infrastructure (blowing up inconvenient rock formations when building roads etc.) might be things they're going to be very interested in trying explosive powder for.
Many early cannon were made of bronze. Would that be within Roman capabilities?