Is one allowed to go to an open casket funeral? Is there an issue with seeing a dead person's face if he's not Jewish?

When the Talmud (Horayot 13b) refers to not looking at the face of a dead person, there is no differentiation there (nor in the reasons given) between Jew and gentile, so you shouldn't look at a deceased gentile's face either. Nevertheless, if there is a reason one must pay his respects there (e.g. social, political or business reasons), one can do as the Netziv writes there, that if you just glance and don't look closely, then it's fine.