So, in my opinion this Pokémon actually really makes sense.
Since, Gastly is a Poison, and both Ghost type. Meaning it can move it's body around.
And make it's head the form of a fist and uses it to Attack.
Looks creepy having a Gastly so near to you.
By now, I'm pretty sure must of you would say: "HaU DO you dO ThONk dAT".
Because, have you ever looked at it's Attack stat?
Well, it's only 35.
So, yeah. That makes up for the theory.
And, if it can learn literally the move Self-Destruct, why can't it do this.
Now why these types of punch moves?
Firstly, Ice Punch, because freezing to death may be a fear to people.
If you are alone (no crowd walking around) it should be colder because nobody shares their body heat.
And your body heat should also get colder when you die.
Freezing because you where so scared.
It's more-so Paralyzing, though people ought to say: "he froze!" and all.
Fire Punch. Some people might be able to be scared by fire because some reason.
Like, your house got caught in fire. Or you are scared that will happen.
And all your important stuff burned with it too. Or, you are scared that that will happen.
Or you burned yourself (making coffee/car-like accident with repairing it or something/bumper).
Heartburn(I'm not sure, but I don't think the acid actually causes your heart to "burn", though it could be taken literally), running away so fast you exhaust yourself, hands sweating....etc
Paralyzing of fear.
Fear of Thunder and Lightning.
Lightning is fast, and so is your heartbeat when you are scared.
I have no idea why they didn't add Shadow Punch because, why?
Maybe so it was a bit more mysterious "and people are scared about what Gastly can do so a smart PokéNerd will find out and is the most intelligent creature on earth".
And also, like I said long above, it's creepy having a Gastly that is gonna attack you near you.