Monatomic usually refers to the ions or gases of single atom molecules, that is they are not "polyatomic" like CO2 which has carbon and oxygen. Common ones include H+, Li+, Na+.
One of the main benefits of gold is that it is highly, though not completely, un-reactive. That's why it doesn't tarnish. Pure solid Gold does not dissolve in your stomach or small intestine and it has no use as a dietary aide. Only soluble gold salts like, gold chloride AuCl, will dissolve in the body and, all of them I know of, are toxic to one degree or another. The toxicity is avoided by adding chelating agents to the mix but that would seem to defeat the purpose of taking a gold salt, unless it is only meant to be a placebo effect. It would therefore seem that the primary use of monatomic gold, as a dietary aide, is to separate the buyer from their money.