According to the v2.0 documentation for the User object gender
and locale
fields should both exist and are considered part of the Core API -
If you're not able to fetch those fields with a user access token for that user, it's likely a bug, but it should work OK in both versions
If you're trying to fetch that data for users that don't use your app, that's unsupported as the primary changes in v2.0 are to apps' ability to fetch data about people who don't use the app