Where did you find the old man in ghost tower Pokemon fire red?

The old man you are referring to in Ghost Tower is actually not in Pokemon FireRed. He is a character from Pokemon LeafGreen and can be found on the second floor of the Ghost Tower in the far left corner.

He is a trainer who uses a Gastly.

While there is no equivalent of this character in FireRed, he is a popular and memorable character in LeafGreen, and it is easy to see why you might remember him from your playthrough.