In case anyone is wondering why the newer builds don't work with the GamePi20 is because the wiki uses fbtft_device and fbflex both retired from the linux kernel starting from 5.4.
Possible solutions:
Make a Device Tree Overlay targeting this display (according to my research it uses a ST7789(v) LCD Controller), the driver is included in the linux kernel so no need to add it..
Use fbcp-ili9341 as it seems to support this controller.
Downgrade the kernel to <5.4 (not sure if possible) and use the installation method given in the wiki.
None of this method have been tested by me since my GamePi20 is yet to arrive, but theoretically it should work.
Update: My gamepi20 arrived and it works beautifully.
I downloaded the lastest Raspbian Lite and downgraded the kernel to 4.9.118.
Made a FBTFT_Device using the steps listed in the wiki and installed FBCP.
Downloaded an old version of raspberry-kernel and raspberry-kernel-headers of around the same time the kernel was build.
Installed DKMS
Build and installed mk_arcade_joystick_rpi from source and apply configurations. If DKMS fails try other versions of raspberry-kernel and raspberry-kernel-headers that includes your kernel.
Added fbtft and mk_arcade_joystick_rpi as modules
Modified config.txt according to the wiki
Installed Retropie using Retropie-Setup
If done correctly you should have the lastest Retropie + RaspberryOS but with an old kernel