What's stopping me from selling (or giving away) a game I purchased on GoG?
Apart from the fact it's illegal, is a violation of GOG.com's TOS and will likely result in your account being closed if they catch you? If you sell a GOG.com download, their system allows you to download it again & potentially sell it on, again. That would make a you a software pirate.
If you are doing what you are insinuating, you're part of the problem and the reason we have DRM-ed services such as Steam, D2D & Ubi-Online (or whatever its called).