A GFCI is designed to trip around 5 milliamps and this is for an approximate safety factor of ten times. Typical figures Ive seen for defibrilation are 30 to 50 milliamps.
Again the voltage output of a buzz box AC transformer welder is between/across its leads (stinger and ground NOT either lead to other AC wiring) but one or both of those leads are against metal or on concrete or wet mother earth and if your body gets between one or the other (or water or metal etc) you get shocked..
Electricity doesnt necessarily do weird things (from an engineers perspective), current simply tries to flow anytime theres a voltage difference and ohms law defines how much in whatever paths are available subject to the paths resistance.
Hang in there, youre comin around