If your house is wired properly, ground and neutral are bonded together at the power entry panel, so there should be no current between the two. A GFCI doesn't monitor the ground and in many cases, you can install them without one attached at all in older installations where there may not be a ground. So, if you were injured by current on the ground OR neutral, it would be a fluke that the GFCI couldn't help you with. You're right, it looks for an imbalence between the hot and neutral, and (most?) trip at about 5ma of current; far less than what it takes to kill or injure a healthy person.