Gas pressure is the force per unit area exerted by the gas molecules as they collide with the surface of an object.
In SPM, especially in paper 2 essay question, you need to know how the gas pressure is produced. (See question below)

Question:
Explain how gas pressure is produced in a closed container?

Answer:

Gas molecules in a container are in constant and random motion.
As a result, the gas molecules collide on the wall of the container.
After colliding on the wall, the gas molecules bounce off, and the direction change creating a change of momentum to the molecule of the gas.
The change of momentum produces a force on the wall.
The force per unit area is the pressure on the wall.