A neutral ground bond links the outside world, building, and metallic surfaces to the neutral return portion of the AC voltage to the energy system, providing a pathway for both ground faults, and a patheay for gfci functionality...
But the GFCI will happily and properly function to protect humans from energy flow outside the gfci device...
In other words, bonding to ground is useful for many safety pathways.
A GFCI device does not need a ground attached to it to protect the hot energy running through the gfci from straying into a person, or leaking via a fault.