Question

what does a gas exert pressure

Answer 1

gas continuously hit the walls of the vessel in which they are kept this is called pressure exerted by gas, i think you can refer to ncert txtbk, class-9 chpt.1

Answer 2

The molecules are continually colliding with each other and with the walls of the container. When a molecule collides with the wall, they exert small force on the wall The pressure exerted by the gas is due to the sum of all these collision forces.The more particles that hit the walls, the higher the pressure