sorry to hear you are having trouble with this ><
If you have access to a PC you can try the following. Mount the first partition of the microSD card to which you have written the Gentoo image (it is formatted with the FAT filesystem, so you should be able to access it even on a Windows PC), and then edit the file config.txt
(at the top level of this filesystem), uncommenting the following line, so it reads:
Leave the rest of the file as-is, save, eject the microSD card and try booting again. Hopefully you will now get a successful boot with a basic, low-res display.
Once running, you can then use the RPi Config Tool (Applications -> Settings -> RPi Config Tool) to choose a more appropriate resolution.
Not sure why the automatic detection is not working in this case (as it does work for the vast majority of monitors, out of the box), but does work OK in PiOS64. If you could dump the EDID data from PiOS, that might help to narrow things down.