Not exactly what you are asking for, but we can have a Lemmy community for that. Feel free to contribute to !gamereviews@level-up.zone and I can help with moderation.
I'd be more bothered by the closed sourcedness of the movies and games themselves, than of the reviewing venue. They're still proprietary media products wherever they get reviewed. So I'd work on that first.
I do like open source games, but that's a very different and unrelated thing to this. Whether the reviewed game is open source does not matter too much from the review perspective.
Are you open source ? I mean you as a person, do you give open access to all your datas by example ? If not, that bothers me. I mean how can you be a hardcore open source supporter, and not be open source too...
Not open-source, but I was using Metacritic as an alternative to Steam. Unfortunately, they've severely degraded their UI in recent years so I started using GOG.