Two Face reveals that he is backing Gordon in the election so that he will be appointed District Attorney again. However, District Attorney is an elected position, not an appointed one, Gordon being elected would have no impact on Two Face getting the job.
Two Face says that Gordon will win by default after the incumbent Mayor dies. This is not how it works. If a candidate dies four days before the election, their name will remain on the ballot and people can still vote for them. If they won, their position would then be filled according to the law in that state. While some voters might switch over to Gordon after their preferred candidate died, others would continue to vote against Gordon because they do not like him.