While it isn't in the same league as the GOG store, the ZOOM Platform sells DRM-Free games only ... not to be mistaken for the ZOOM conferencing app and site.
Currently it has 8 free games, four of those are also free at GOG, one isn't, and three don't exist there.
Bio Menace (GOG) - https://www.zoom-platform.com/product/bio-menace
Death Rally - https://www.zoom-platform.com/product/death-rally
Killing Time (GOG but not free) - https://www.zoom-platform.com/product/killing-time
The Monuments Of Mars - https://www.zoom-platform.com/product/monuments-of-mars
Postal 1: Classic & Uncut (GOG) - https://www.zoom-platform.com/product/postal-1-classic-and-uncut
Shadow Warrior: Classic & Complete (GOG) - https://www.zoom-platform.com/product/shadow-warrior-classic-and-complete
StarGunner (GOG) - https://www.zoom-platform.com/product/stargunner
Warzone 2100 - https://www.zoom-platform.com/product/warzone-2100