The 'tater digger was gas operated like most modern machineguns. It lost out in the early days to the recoil operated guns, which were considered much more reliable and less likely to foul with the dirty, corrosive ammo of the times. The Lewis suffered the same problems. Later, the gas operated Bren reached perfection, but only after the adoption of stainless steel in the parts subject to combustion gases. Note the stainless cylinder on the Garand. (One point favored on the Johnson rifle was it's recoil operation)
The real machineguns of WWI were the Vickers and Maxims, with the Brownings arriving at the very end. All known for reliability and firing 10's of thousands of rounds between stoppages; and all recoil operated.