So TWRP does not work on PIxel 3 XL with Android 10 or 11 but I also found out GSI does not flash also on Android 10 or 11. So make sure to downgrade to Android 9 when trying to install GSI ROMs otherwise you will get error with fastboot.
This could be an issue with Pixel 4 series also!
Also you need to disable verifcation by:
fastboot flash vbmeta vbmeta.img
BEFORE even trying
fastboot boot twrp.img