SignalFrogGold has one property no other common asset shares: it is nobody's liability. A share depends on a company, a bond on a borrower, a bank deposit on a bank. Gold depends on nothing.
That is exactly where its role comes from — and also its limit.
What it has protected against
Against a loss of confidence in currencies. In periods when the purchasing power of a currency was openly in question, gold did its job.
Against negative real rates. Gold pays no interest. That drawback disappears when safe assets yield nothing after inflation either — historically the phases in which gold was strongest.
Against systemic events in which counterparty risk itself becomes the issue.
What it has not protected against
Ordinary equity corrections. The relationship between gold and equities is historically weak and changes sign; there were declines in which gold fell too, because positions were being liquidated there as well.
And against inflation in the short run — over decades gold has preserved purchasing power, but across individual years the relationship is weak enough that it cannot be relied upon.
How it fits
As an admixture that behaves differently from the rest — not as a reserve. The reserve is what still holds its face value at the moment you need it. Gold can be twenty percent lower at exactly that moment.