There are these thingswe wused to put on DVDs, we call them install files. You can find them on like every torrent tracker these days.

In the case of games, anything that doesn't mean losing access to an account stops you from being able to play games you already bought. So, GOG wouldn't need to change much, but Steam might. Wouldn't have to mean shipping physical media.

Or they can just stop using terms like "buy" and otherwise implying ownership, instead making it clear you're just leasing the game.