And it's not just key metal deposited on the screen, there are deep grooves cut into it.
Your brass keys should only be about 3.0, and gorilla glass a 9.0- you should be able to rub keys into brass dust all day long and not mark the screen.
I store my keys right in my pocket with my glass iPhone SE, and it doesn't have a mark on it.
I suppose it is possible you have keys with some weird material or coating, or as another poster suggested- maybe something like an abrasive dust got stuck on your key.
Typical keys are made from nickel plated hardened steel and nickelsilver. Considering the shiny coating is long gone and the keys are ferromagnetic, I assume it's the first. The keys are certainly hard enough to scrape my glass ceramic stove.
And reports online say that gorilla glass victus is the mist scratch prone gorilla glass, scratching at or around 6 already :/
My Pixel 4a, meanwhile, was in my pocket all the time and never got a scratch.