Gold standard usually refers to a diagnostic test or benchmark that is the best available under reasonable conditions.
Other times, gold standard is used to refer to the most accurate test possible without restrictions.
Both meanings are different because for example, in medicine, dealing with conditions that would require an autopsy to have a perfect diagnosis, the gold standard test would be the best one that keeps the patient alive instead of the autopsy.
The phrase is therefore ambiguous and its meaning should be deduced from the context in which it appears. Part of the ambiguity stems from its usage in economics, where “gold” does not imply “best” but is merely one of many possible standards.