GSIs (Generic System Images) are a type of firmware that does not modify the kernel (boot.img), as opposed to vendor firmware. This allows a single GSI to run on many different devices, and it allows for custom firmware to run on devices where the kernel sources are not available (which are required for vendor firmware)

To run GSIs, the device needs to support project Treble. You can check support and see what kind of GSI you need with the TrebleInfo app