Hi, GRCI's are not as mysterious as they sometimes are thought of they are very simple in theory. All they do is measure The power (amperage) that goes out on the Hot wire and then what comes back on the neutral or white wire. That is the slightly wider prong on the plug. If there is any difference (4 to 6 Milliampers) it trips the receptacle. It figures the lost amperage has gone to ground.
Good common sense should always prevail. Any damage to the cord should be repaired prior to use.
Saying this is like having a warning label on a gun barrel or on a pack of cigarettes.
Just to get a feel for what low levels it trips at you may feel 1ma (a tingle) at 4-6 the GfCI trips and at 10 ma you can still let go but at 15 ma you hang on and it goes on from there. Most of us will survive a encounter of 30ma. From there up amperage causes damage .Remember that electricity is harder on children than adults.
I hope this helps Jted
Edit of the post.
I just read WaOauu post. I guess we were writing at the same time. He is correct It can sense moisture but that is not it's main function.
I heard 6ma kills but it depends on which appendage is hot and which is grounded. Like right hand to left hand across the chest is the worst.