Gengar is such a popular Pokémon that I would be surprised if it didn’t get added.
Not to mention that I’m pretty sure it has appeared in all the main series games.
I played the original Pokemon Snap game years ago on my cousin's old N64 with her. I didn't realise how to complete the final level until years later, though. It's not one of my favourite spin-off games, but it was sill a good game none the less.