Guide

How to flash GSIs that are image files

1)Download GSI outside of the phone(like your pc, or sd card).

2)Flash the STOCK fastboot ROM of your phone.

3)Flash TWRP(BRP recommended).

4)Wipe only system&data.

5)Unpack the zip file of GSI, you will find the image inside. 

6)Copy the image to the phone.

7)Go "Install" and tap "Install Image".

8)Flash the image that you copied.

9)Once it's done, patch vbmeta, and also disable recovery replace(from advanced menu of BRP).

10)Reboot and hope that it boots fine.