I'm a geophysicist, I have a lot of experience with magnetometers for mineral exploration. We're looking at using one for this.
There are some buried utilities and we don't know what the gold bars are encased in, it would be nice if it was a ferrous metal container. They seem to think it's a suitcase or something like that but we don't know.
The utilities and proximity to power lines, septic and other things will probably make the mag difficult to use in this case. I have a proton mag and we will try it but I have have doubts due to the potential interference.