It will be combustion of methane, because combustion will have the reactant and the oxygen, producing carbon dioxide and water, which is whats happening in this case.
Gunpowder is a heterogenous mixture,
Reason : the composiotion of the mixture is not uniform throughout, the mixture can be separated by a suitable method , and finally , it composes of charcoal, sulfur and potassium nitrate which is not homogeneous mixture.
