I have not seen a dog with gunpowder ingestion myself but have been looking at some literature for you on the subject. It appears that the nitrates in the powder can lead to blood changes and effective anaemia, but if the gunpowder is in a reasonably high quantity. The gunpowder formulation itself may be irritant to the inner walls of the stomach and intestines and so it may well cause vomitting and diarrhoea in your dog as a minimum.
If you do not know how much has been eaten, then taking her to the vet would be your best option, as you can then be sure that she can have all the supportvie therapy she needs for if there is a potential toxicity. 3 hours is a little too late to acheive much by inducing vomitting so they may want to flush the stomach, and provide fluid support, as well as checking the blood too. Signs of toxicity can still develop after these three hours.
If the vet is not an option, then keep a very close eye on her to check for any signs of severe vomitting, diarrhoea, weakness/wobbliness and trembling as these can be signs of toxicity. Feed something bulky and bland now such as some bread to try to help the gunpowder to pass through the guts and offer plenty of water to drink. Do not be suprised if diarrhoea develops - if it mild then you can use bland food sich as chicken in small and frequent meals. However, if this is persistent or she seems poorly at all, please, please go to the vet - the prevention of toxicity is much better than treating the signs after they have developed.
I have enclosed a link to a good webpage to give you a little more information:
I hope this helps,