Has it really been that long? Apparently so. Valve originally announced their rebranding of Steam Play with Proton back on August 21st, 2018. Seems like a good time for a quick reflection being halfway to a decade old now with the tech that gave rise to the Steam Deck.
I was aware of the Game Porting Toolkit announcement. In fact, I first learned of it from CodeWeavers, who noticed Apple used their code to develop it.
I don’t play games on macOS, but my understanding was the same as yours; that it was a testing tool for developers to test out how the game might work on macOS. That’s how it was presented. I didn’t think it was meant to be used by macOS users.
In any case, CrossOver is intended to be used by macOS users (and the GPT uses the same code, as enthusiastically explained above), and it has a good graphical interface. I think these factors make it obvious to recommend CrossOver as the canonical solution for playing Windows games on macOS.