P.S. — What I hadn’t considered before was that tripping the GFI didn’t involve the hot (120V) wire at all. I had previously checked by opening the main breaker, and also confirming that hot wire wasn’t grounded at all, but even then I kept tripping my GFI at home. It makes perfect sense now, but it took someone else to point it out. I kept looking for current flow from hot to ground, but that wasn’t the right place to look.