Any gas is a collection of innumerable number of minuscule particles which are known as molecules according to Avogadro’s law.
There are no forces of attraction or repulsion among the particles or between the molecules and the surroundings.
The gas particles are always at straight, rapid, fast & random motion resulting in inevitable collisions with other particles and the surroundings that changes direction of motion.
Since the particle are spherical, solid and elastic the collisions involving them are elastic in nature as well i.e their kinetic energy is conserved even after collisions.
The total kinetic energy of the particles is proportional to the absolute temperature.