Your misunderstanding me. Ik why Gengar was originally there. I'm saying the way Gengar acted, you could just replace his backstory with him just randomly haunting the lab and nothing will change.If you rewatch Gengar's first few episodes, it haunting the lab and the professors house was a direct result of it being abandoned.
They don't indicate he pranks ash and the others because of trauma
They don't say anything about the abandonment after the capture episode
That's why I said if you take out his old trainer, nothing will change