If you get stuck in a GSI bootloop or stuck in fastboot where it says, “reboot bootloader” in fastboot mode, then you are most likely getting boot verification error. You MUST flash vbmeta.img which disables the boot verification so you can boot your GSI.
Go ahead and download this:
and flash it with fastboot like this:
fastboot flash vbmeta vbmeta.img
And your GSI will boot.