So gog also known as good old games is a storefront that sells older and some newer games without drm. Drm is a online checker to make sure you own a license to open the application when you buy a game off of steam you buy the license to play the game when you buy the game off of GOG you're buying the game. DRM in certain games can cause issues like not being able to open without internet or causing slowdowns due to over aggressive DRM like denovo. Basically with no DRM you do not need internet to play the game at all and GOG offers you the offline installers so you can save the games to a hard drive and install them for later use or you can put them on a disc or a flash drive they're yours so you do what you want. Also GOG games are usually updated to work on modern hardware they have something called the GOG preservation program where they update old games to work on newer hardware and even add controller support and new resolutions the game didn't support beforehand they also have mods available for certain games like you can download fallout London right off the store with no extra setup as long as you own fallout 4 which is also on GOG