Outlet Mystery

Hello all, I'm a new poster so hopefully I provide enough information for the forum readers. We have a house that was built in the middle part of the last century and has some wiring "quirks". All of the outlets in the house work fine except for the one in the bathroom where we aren't able to hook up a GFCI. The wiring coming into the outlet box is Romex 12/3 and here's what I find. (Also, I'm guessing that some of the voltages that I'm seeing are from my digital multimeter) Neutral to Ground - 16V Hot to Ground - 16V Neutral to Hot - 0V