While an alternative except GOG doesn't really exist. What you can do is buy your keys from any site linked by https://isthereanydeal.com/
These sites sell keys but money still goes to the developer, its just that steam does not get a cut and the companies on there do.
The companies that I know for a fact are EU based Greenman and Fanatical (UK), Gamesplanet (France)
There's also GamersGate (Swedish)
Its already been mentioned but if you don't need reviews you can use Here WeGo maps. Its a good alternative and with more users (who make fixes if something is missing) it'll get better and better