Stuck at boot animation logo after flashing GSI image system

Maybe you forgot to flash miui rom then the gsi files/custom roms (xiaomi miui supported device only).
Method 1 :
Before flashing GSI files using fastboot first goto twrp and wipe data, cache , dalvik cache then flash the Miui rom for your phone after that flash GSI File using fastboot for your device.
Note: after flashing miui ROM don't boot to the os boot directly to the bootloader then flash the gsi file using.

fastboot flash system name-of-your-rom.img

Method 2 :
You can flash miui fastboot rom for your device using fastboot then flash gsi/custom rom
After flashing fastboot miui ROM for your device reboot to bootloader again using

fastboot reboot bootloader

Then flash gsi/custom ROM file using.

fastboot flash system your-system-image-name-here.img

Method 3 :
If you're flashing a custom rom compiled at .zip format you need to wipe system, data, cache partition then flash the miui rom after that flash the custom ROM you want. You don't need to restore your stock boot image when you've installed a custom ROM of course it won't boot and sometimes it does . In some device you need to patch the dm-verity on boot.img if you're installing twrp on a stock rom and flashing magisk doesn't fix your problem.