DRM free does not mean you own the game. All companies, including GoG rely on the 'licensing trick' to avoid granting first sale doctrine related rights, like reselling. You can read the EULA in any game you buy from GoG or read part of it here: "3.3 Your GOG account and GOG content are personal to you and cannot be shared with, sold, gifted or transferred to anyone else." [1]

Otherwise yeah, you'd be able to buy most stuff on GoG for pennies by now.

[1] - https://support.gog.com/hc/en-us/articles/212632089-GOG-User...