Work done on a gas when it is doing work?

If a force moves in the same direction as the force itself, then it is doing (positive) work. So if a gas pushes a piston outwards, we say that the gas is doing work on the piston, and therefore on the surroundings.

But, you say, the surroundings are exerting a force 'inwards' on the piston. When the piston moves outwards, the force exerted on it by the surroundings is in the opposite direction to the direction of motion of that force. The surroundings therefore do a negative amount of work on the piston. In other words, the surroundings have work done on them – exactly as we concluded in the first paragraph by considering the work done by the gas!

During expansion, the gas exerts a force on its surroundings in the direction of the displacement of the piston and the atmosphere. When force is in the same direction as displacement, the work being done by that force is positive. Positive work means the source of the work (the gas) is transferring energy to the surroundings.

Now looking at it from the perspective of the surroundings (piston and atmosphere), it is exerting a force on the gas that is opposite the direction of the displacement. Consequently, the surroundings is doing negative work on the gas. When negative work is done the source of the work (surroundings) takes energy away from the gas.

For conservation of energy, the energy transferred from the gas to the surroundings due to positive work equals the energy taken from the gas by the surroundings due to negative work.

Hope this helps.