The best free alternative to GOG.com is Steam. If that doesn't suit you, our users have ranked more than 50 alternatives to GOG.com and many of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to GOG.com are itch.io (Free, Open Source), Epic Games Store (Free), Desura (Free) and Instant Gaming (Free).
Indie gaming is coming to mobile! With this app, you can easily install Android games from itch.io and keep them updated. This provides an alternative to the Google Play store for indie game developers and enthusiasts.