Beginners using vanilla GNOME
Beginners will never really be in a position where they'll be using vanilla gnome, so that argument is kinda moot. And even if they did, those features are literally one extension away...
will quickly miss features like a minimize button and certainly tray icons.
Tray icons don't exist in gnome's ecosystem, it only becomes problematic once you get third party applications. The real problems are the minimize/maximize, desktop icons, and panel on top when coming from windows. Although these days with the ever increasing phone use people might just be more at ease with gnome's workflow anyways.
I'm not really sure I am... Do we have some actual numbers into how much money they've sunk in linux?
Gaming on linux is a huge community effort, whether it's wine, dxvk, vkd3d, mesa, linux itself... and plenty of smaller projects like lutris, bottles, UMU... And all this spans literal decades, far before valve ever got involved.