I'm not training the Gastly so if you want one, you'll have to request one on the GTS. That being said, Gengar is my favourite Pokémon too and I get that you might not want to risk losing it in a trade with some stranger. If you want, feel free to contact me once YOU have evolved the Gastly and I'll help you with the trade evolution directly.
Edit your post if you want the Gastly.