The rating is calculated by the report of the users, there is no manual rating done. GTAV works for most people, but some, so it ends up being Gold.

However, these Medal ratings are carried over from the WineHQ rating, when ProtonDB was nothing more than a spreadsheet on Google Docs and was also used when Ratings were considered by the reporters itself. But this turned out to be a bad idea, as people tend to give platinum ratings, despite naming issues in their reports.

So the attempt was to remove the manual rating and instead ask the users about their experience with several talking points and consider by that a rating. This would still use the old medal system, which of course does not work well for such system. That's why a new tier system, which is more a representation between very good and very bad, was created, and you can see it on the dashboard (Change the Rating System to ProtonDB Click Play). However, this was considered as an internal test, but then the main developer was pulled away from ProtonDB for other work and all went basically to a hold.

Now the developer is back at work on ProtonDB and this will hopefully lead to a new and better rating system.

I hope this will shed some light on why it is what it is, and that no one is really happy about it.

Source: I'm one of the administrators on the ProtonDB discord.