Apple Gaming porting toolkit 1.0.4 : Steam using Whisky
Despite following carefully the instructions (you have to ensure x86 homebrew is installed/used instead native arm, not the same paths) I have hard time making it work : since Steam is installed, next time it is launched (or a game is downloaded and launched) there is a message stating D3DMetal required MacOS Sonoma and an Apple Silicon Mac (which both I have !). I followed many tutorials, that are in fact based on Apple GPTK image readme, plus some tricks and game compatibility list. I also tried using Wineskin server/Winery with latest D3DMetal engine, or even Whisky latest build (1.1.1, August 24. It allows to enable/disable quickly D3DMetal - works only with DX11 and DX12 games - or DXVK - for older/DX9 games or those incompatible/GPTK)
I used the free UTM virtualization solution (that is based on Apple's hypervisor framework), it installed Sonoma b6 really fast on the container (on internal ssd) and I allocated 8 (powerful) cores and 20480 MB memory. All is very responsive (despite mouse scrolling sometimes lagging and reverse scroll option in UTM not working). Also you shouldn't move the container (in user/Library/Containers/UTM) to an external disk, as the virtualized Sonoma is since very slow (despite using an external Samsung 980 Pro nvme4 ssd in a Thunderbolt 4 enclosure). In fact there isn't a Sonoma beta installer available, only the ipsw restore file (which we used/dragged to UTM to install). So if you prefer installing Sonoma beta on an external drive (instead VM), you have to donwload Ventura 13.5.1 installer first, then install Ventura on the external drive, then boot into it, and finally update to Sonoma using settings panel. It was said however recent MacOS aren't made to be run on external drive.
To note, compiling GPTK (using brew) took close to one hour on MacOS Ventura (my first try without VM), and twice the time on the VM (the Mac mini enclosure was severe hot during the process - the 8 cores were used at 100 percent).
Then installing Whisky is fast, in fact you just have to drag the GPTK image on it and create a Win10 bottle (located in user/Library/Containers/Whisky/someId/name), and I found a solution to steam problem here :
https://mybyways.com/blog/running-steam-in-game-porting-toolkit
You just have to add these config arguments (step2) : -allosarches -cef-force-32bit
I didn't encounter/had to fix Blank login dialog (step1) nor Steam blank page (step3).
However, still not working, launching Myst (DX12) states application requires Sonoma and Apple Silicon !