In the unlikely event that a person contacts both the hot and neutral, and is otherwise insulated from the surrounding environment, a GFCI will not protect against that shock.
One scenario where it could happen is if a "grounded" device is plugged into a bootleg ground receptacle, and another "grounded" device is plugged into an RPBG receptacle, and the person contacts the chassis of both devices. A GFCI upstream may not protect from that. Of course, it's likely that the GFCI would trip well before that happens (because of the bootleg grounds), but hey, it's a conceivable problem.
Perhaps a few too many "and thens".
I ASSume you mean bootleg ground to neutral (not RPBG). If the GFCI outlet and normal(?) bootleg outlets are used in parallel (not plugged into each other), the bootleg safety ground connection is effectively on the input side neutral of GFCI protected outlet. In that scenario any line to safety ground (to neutral via input side bootleg) fault current is outside the GFCI loop and will trip the GFCI outlet at the expected >5mA.
As I've warned before, if a GFCI bootleg ground is connected to neutral at the output side of the GFCI, that will defeat the protection for faults to that bootleg ground. If it is bootlegged to the input side neutral the GFCI will work, but still DON'T DO THAT it is always unsafe practice to bootleg.
Better IMO to just float the safety ground and let the GFCI do it's magic.
JR