Update: It IS possible to install Lineage OS GSI on the G8 Play. However, you will trigger a security error (nothing critical, since your phone will still boot up). To install it, you really need to flash the One Macro Firmware (evidently, your device can hard brick, but it's possible to fix it ). Later, you can flash the Lineage rom through fastboot.
I already tried that on the official ROM and it didn't work. I only get a DM-Verity error amd a bootloop when trying to boot Lineage, which can be solved if flashing the stock ROM again.