Last I checked, TWRP couldn’t flash logical partitions that make up where the GSI is patched, but it was requested, maybe something changed? Idk, the guy who created the guide on how to flash GSI on my device’s XDA forum mentioned this issue a while ago and recommended flashing through ADB.
Actually, I think the issue is that when I flashed my device, TWRP couldn’t unencrypt or something, it’s been a while and I know I should start the process from scratch