Is a gas leak considered incomplete combustion?
In climate science we call these leaks fugitive emissions. The chemical composition of the emissions are different. If you just have a leak, the emissions will consist of what ever your fuel is made out of, probably methane. If you burn your fuel and the combustion is not perfect you might sill have a littel bit of the original fuel left but most will have been used up and formed CO (incomplete combustion), $\ce{CO_2}$ (complete combustion) and $\ce{H_2O}$ .