Huge difference actually. On the surface, the technology/basic layout of the engines that you work on differ between the conventional piston engines of the EN which are usually Diesel now and the Gas Turbines that the GS Rating(s) were created for. That is a huge difference in terms of types of maintenance, level of maintenance that is possible at the organizational (ship) level and operating conditions. Drilling down a little deeper though there are often even larger differences based on ship type. On a few ships, primarily Amphibs and Minesweepers, the Main Propulsion is Diesel and thus, there will be quite a few ENs of varying levels because they operate and maintain the ship's main power and propulsion system. On many other ships such as Destroyers and Cruisers, the Main Propulsion is Gas Turbine and there, there will be many GSs but also ENs for the Emergency Diesels as well as Boat Engines and a variety of other equipment.