What creates gas pressure in a container such as a helium balloon?
Pressure is rigorously defined as force per unit area.
So the gas pressure of a helium balloon arises from the impact of the collisions of the helium atoms between themselves and with the inside surface of the balloon.
Of course, the outside atmosphere similarly exerts a pressure on the outside of the balloon. However, because the density of helium atoms is greater than the density of air molecules, the balloon wins and its volume expands with respect to the atmosphere.