There is no way to debug installation with adb operation mode, install Shizuku, start it, and grant Shizuku permissions to DSU Sideloader, after that, you should be running as Shizuku operation mode, now, try installing a GSI again.
i had the same issue and this shizuku helped me debug the issue. apparently dsu installation tool was trying to access sd card and was getting permission denied. it worked fine after i removed the sd card
There is no way to debug installation with adb operation mode, install Shizuku, start it, and grant Shizuku permissions to DSU Sideloader, after that, you should be running as Shizuku operation mode, now, try installing a GSI again.