When a Pokémon is evolved into a new Pokédex entry, 500 experience points are granted on top of the usual 500 for evolving a Pokémon. Do these points also get awarded for a different gender evolution of the same Pokémon type, which appears as a silhouette in the evolution button?
No. You only get the bonus 500 experience for the first time you catch or evolve a Pokémon, regardless of gender. You do not get 500 experience for each variant of a particular Pokémon, as they are all part of the same species.
Case in point, I just evolved a Gastly into a Haunter for the first time since the Pokédex update: