I am able to revert back to stock without problems and also got Havoc GSI installed. Will keep this thread posted on how it performs.
Below is what I did. Some of these steps may not be required. I don't know which.
1. Download a AB GSI. I used Havoc-OS-v3.6-20200613-Official-GApps-arm64-ab.img
2. Flash the stock ROM. I used UMIDIGI_Power_3.E_V1.2_202003282206.
3. Boot into the stock rom. In developer settings enable OEM unlock and bootloader unlock.
4. Perform the below commands in adb
5. adb reboot recovery
6. Select Factory reset and reset.
7. adb reboot bootloader (or use the menu option)
8. fastboot flashing unlock
9. fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img (from stock ROM)
10. fastboot --disable-verity --disable-verification flash vbmeta_system vbmeta_system.img (from stock ROM)
11. fastboot --disable-verity --disable-verification flash vbmeta_vendor vbmeta_vendor.img (from stock ROM)
12. fastboot -w
13. fastboot reboot fastboot (you should get to fastbootd menu)
14. fastboot erase system
15. fastboot delete-logical-partition product
16. fastboot --disable-verity --disable-verification flash system Havoc-OS-v3.6-20200613-Official-GApps-arm64-ab.img
17. Reboot to system.
Thanks to @mrmazak and a few other people on the internet. I just did this by trial and error. So, unable to help you with any troubleshooting. If you follow the above steps, things should work.
6/30: The above process works for all AB GSIs. However, getting root is an issue in all of them. There are modified magisk versions available, but no way for us to flash it. You can't flash it from the unofficial twrp because it says that system is not found. From stock recovery, sideload doesn't allow it to be flashed either.. so, I'm back on stock for now.