This doesn't seem to be OP fault, just stock Qualcomm bootloader.
WARNING This is still ALPHA
Correct installation procedure of a GSI on the One Plus 6/6t.
Make sure you unlocked the bootloader and that you have the absolute latest ADB and Fastboot from Google, none of the minimal packages we know of will work!
Then in fastboot flash the following, WITHOUT rebooting in between! this needs to be done all in one go!
fastboot flash boot_a boot.img fastboot flash boot_b boot.img
Use the STOCK boot image!
Then flash the GSI system image of your choice
fastboot flash system_a system.img fastboot flash system_b system.img
Then flash STOCK vbmeta image with the specific commands!
fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img
then just fastboot reboot or select start on the phone.
If the above fails and you get a bootloop you will have to repeat all the steps above.