[Xiaomi Mi 8 Pro Guides, News, & Discussion] nealed:
The below is a guide to installing GSI images on the Mi 8 Pro. I specifically have experience with the PixelExperience 9.0 (Unofficial) but have had other GSI ROMs booting and working. I hope this is useful for some.

GSI (Generic System Image) ROMS can be flashed to any Project Treble enabled device. These devices have either an A/B partition layout (these allow for seamless updating as the system can boot from A or B partition) or just an A only partition layout. The Mi 8 Pro is an arm64 a only device and support project treble ROMs. You can choose from suitable ROMs listed here and here.

The instructions given below are for Android Pie (9.0). The same basic setup can be used for Android Oreo (8.0, 8.1) ROMs, but the Mi 8 Pro should be flashed with the appropriate Xiaomi base ROM first before flashing any 8 based GSI ROM

As the ROM development for the Mi 8 Pro is basically dead, I decided to buy a OnePlus 6T and sell my Mi 8 Pro. However, due to having my new OnePlus 6T stolen, and due to a lack of funds to buy a new device, I have revived my Mi 8 Pro to try to get something other than MIUI working on it. I now have The PixelExperience (Unofficial) GSI ROM running very smoothly on my Mi 8 Pro.

As the PixelExperience team has stated they will be delivering GSI images again, if there are enough of us asking, we could get better support for things like the fingerprint sensor, for a more complete experience.

Some of the below steps may be unnecessary, but are there for a complete beginner's guide to doing this and getting the right results.

Working:

WiFi

Bluetooth (with APTX)

NFC (Including Google Pay)

RIL

Mobile data (LTE / HSPDA / 3G / EDGE)

GPS

Camera (with camera built in to ROM, as well as some GCAM implementations)

Video Camera (not working on GCAM implementations tested so far - only working on Camer built in to ROM)

Flashlight

Sound / Vibrations

Face Unlock

Hotspot

Not Working:

VoLTE (I have had it working, but unable to end phone calls and problems receiving SMS messages, so reverted)

Fingerprint (There is some support. The fingerprint area of the screen lights up when trying to enrol a new fingerprint. The screen dims when a finger is placed on the sensor, and a green light us emitted from the sensor area, but it does not register the fingerprint and this is not currently working.)

Notification LED

You Tell Me

How to install

Flash the latest Fastboot Global Stable Android PIE (9.0) ROM for the Mi 8 Pro. (eg. equuleus_global_images_V10.3.4.0.PECMIXM_20190712.0000.00_9.0_global_9a181f79c1.tgz) using the Xiaomi Tool.

Update: Use the latest MIUI system. The contents of the vendor partition are important. The most compatible vendor partition as of 2019-08-19 is from equuleus_global_V10.3.5.0.PECMIXM. You can download and flash that vendor independantly, if you have already set up your GSI image.

Reboot and finish basic MIUI setup.

Reboot to recovery.

[*]Format data (Wipe - Advanced - format data - confirm by typing yes)

DO NOT FORMAT DATA. Just perform a factory reset. Formatting data can cause issues and prevent booting.

Wipe Cache - Dalvik Cache and System

Reboot recovery

Use adb to push PixelExperience GSI image (img file extracted from download) and latest version of Magisk to root of SDCard. (Empty due to formatting data).

("adb push c:\path\system.img /sdcard/" and then "adb push c:\path\Magiskv19.3.apk /sdcard/")

Flash the image to system partition by taping INSTALL and then on the "Install Image" button (to toggle between flashing zip and img files). Do not reboot yet.

Flash magisk zip in the normal way (hit the same button as in previous step to show zip files to flash. (This is necessary to avoid bootloops)

Reboot, and enjoy.

You can now set up your "PixelExperience" device with your apps and settings, including registering a face for face unlock.

To pass cts validation for the safetynet and to allow service such as Google Pay to work

In Magisk, under settings - Find “Magisk Hide” and toggle the switch on

Magisk restarts. Go to Magisk Hide from the Magisk menu and ensure that...

Google Pay

Google Play

Google Play Services

Google Service Framework

Any Banking Apps

...are checked so that they cannot see Magisk (canont see phone as rooted).

Now go to downloads and install the SafetyPatch v3 module (created by hackintosh5).

Reboot.

Open settings - go to apps and notifications - and find the above apps to clear all data.

Enjoy your rooted safetynet compliant, as close to stock as possible Xiaomi Mi 8 Pro.