The character in this book is a simple-minded savant who has a knack for making things as he needs them. In the course of the book, he invents a device that draws gold from sea water (or any substance the device samples). By the end of the book, a nuclear war begins, and he invents a device that creates a force field that repels the enemy's missiles and sends them back to their starting point.
I suspect that this will be the Bud Gregory stories by William Fitzgerald (actual name William Fitzgerald Jenkins), with his usual pen name of Murray Leinster used when it was published as fix-up novel Out of This World.
The more widely known Bud Gregory series comprises three 1947 Thrilling Wonder Stories stories, as assembled in Out of this World (fixup 1958), plus "The Seven Temporary Moons" (February 1948 Thrilling Wonder); all originally appeared as by William Fitzgerald. Bud is a hillbilly whose intuitive knack with high technology allows him to solve various superscience problems.
From "The Seven Temporary Moons":
.... Once he'd made a gadget which stopped bullets — and guided missiles — and then threw them unerringly back from where they started from. And he'd made a device which was a sort of tractor-beam which drew to itself selected substances only.
A bit of iron at one end of a curiously-shaped coil made the device draw to itself all iron in the direction in which it pointed. But a bit of gold in the same place made it draw gold .... Murfree was using that device, now in Ocean Products, Inc.'s Maryland-coast establishment, and Dr. Murfree was getting much more than ten dollars a day from it.
My initial search of
"idiot savant" gold "seawater" led me to the mention at the SpaceBattles forum, and when I realized they were referring to "Avalon Books", not "Avalon Press", a further search of
"avalon books" hillbilly led me to the Wikipedia entry for Out of This World and then it was subsequent searches for Leinster and the book names that led me to the Leinster entry that described the series, and a possibly illicit (and thus not linked) PDF copy of "The Seven Temporary Moons" from which I got the quote.