How do you catch Gengar in the old chateu and is hidden portrait with red eyes Pokemon diamond?

To catch Gengar in the Old Chateau in Pokémon Diamond, you will need to have the Pokédex item called the Odd Keystone. Once you have the Odd Keystone, you can go to the Old Chateau and use it on the special door on the second floor. This will open the door and allow you to enter the Old Chateau's basement, where you will encounter and battle Gengar.

It is important to note that Gengar can only be found in the Old Chateau during certain times of the day: between 10 PM and 4 AM in-game time. Additionally, Gengar has a relatively low encounter rate, so it may take some time before you are able to find one.

As for the Hidden Portrait with red eyes, this is not a Pokemon in Pokemon Diamond. There are no Pokemon with red eyes that can be caught in the Old Chateau.