Dolarhyde is bewildered by the aliens wanting gold; "What are they gonna buy?" Answer: gold is an essential component of their gravity tech(and any weapon that Blows Enemies Across The Room violates the conservation of momentum, so their weapons are also gravitic). When they abduct Lonergan's girl, the gold coins on the table melt and fly into the air. Their method of mining for gold involves gizmos that do just that, but why would they be using said gizmos on a little house on the prairie? Any gold within range of the gravity effect will melt and float, similar to magnets magnetizing ferrous materials in contact with them.