The user's attacks do 25% more damage to Pokémon as the same gender as the user; the user's attacks do 25% less damage to Pokémon of the opposite gender as the user. If either Pokémon has no gender, there is no change in damage.
Hidden Power's power and type vary depending on the individual Pokemon using it, ranging between 30 and 70 power. There is no way to change a Pokemon's Hidden Power type or power, so you are stuck with what you have. Hidden Power is always a Special-based move.