A Gorilla can punch very hard.
The average force of a Gorilla Punch is between 1300 to 2700 lbs of force.
That's a lot more force for a punch than an human can punch at.
The average punch force for an untrained human is around 150 lbs while a trained human boxer has around 750 lbs of force.
So a Gorilla could punch out even the best trained human boxer of they were to get into a boxing match.
It is believed that a gorilla punch is strong enough to shatter your skull with one slam of its arm:/Between 1300 to 2700 pounds of force.
Gorillas on (avg. 400 lbs) have a muscle mass density almost 4 times higher than the most heavily muscled powerful human you know.
Given that bone density and muscle mass of gorillas far exceeds that of humans, a deliberate punch (say, to the head) from a gorilla would probably kill a mere human.
However, gorillas are typically very timid primates and not very aggressive at all, such that they must be provoked to violence.