Since work done is the product between force vector and displacement vector between two bodies. And the force of gravity acts on an object in a downward direction towards the ground and if a block is moving horizontally on the ground then, it covers displacement in a horizontal direction whereas the force of gravity acts in a vertically downward direction, which makes the force of gravity vector and displacement vector perpendicular to each other which makes work done by the moving block is zero as W=FScos90°=0 Hence, work done by force of gravity is zero.