I've had trouble installing a GSI on my HTC U12+, but AOSP GSI maintainer phhusson helped me, so I'm sharing a quick tutorial.
1. Download and extract Android SDK Platform Tools
2. Download the GSI of your choice (ARM64, AB variant), and move it inside platform tools folder (I've only tested AOSP 10.0)
3. Download and extract the HTC RUU Decryption and Extraction Tool
4. Download the RUU for your device and move it inside HTC-RUU_Decrypt_Tool_XXX folder
5. Follow instructions from the RUU Decryption Tool thread to decrypt and extract the RUU, with all default settings
6. Move the vbmeta.img file you've just extracted (it's inside HTC-RUU_Decrypt_Tool_XXX/OUT_XXX/firmware) to the platform tools folder
6. Unlock bootloader
7. Restore stock firmware with microSD method or factory reset if you already have stock firmware
8. Reboot in bootloader
9. From a computer, launch a terminal, change directory to the platform tools folder, then execute "fastboot erase system"
10. Then execute "fastboot flash system <your_gsi.img>"
11. Execute "fastboot reboot bootloader"
12. Execute "fastboot flash vbmeta vbmeta.img --disable-verity --disable-verification" (WARNING : According to 5m4r7ph0n36uru, you can't flash vbmeta via fastboot if your device isn't S-OFF, you have to use TWRP, but I don't know how, so ask someone who knows! I will update my tutorial once I see instructions.)
13. Execute "fastboot reboot"
It should boot the GSI!
On first boot it may ask for a previous decryption password and complain about a corrupt data partition. You can erase it (you will lose what was inside).
If you have bugs or issues with the GSI, there may be workarounds here: https://github.com/phhusson/treble_e...i/HTC-U12-Plus
Enjoy your GSI on your HTC U12+!