That is determined by the published programme terms, which should be read directly. Family coverage and any per-person amounts are matters for the official material, not for inference from other categories.
Do not import family rules from other classifications
Family treatment varies sharply between categories and cannot be assumed to carry across. An L-2 spouse is employment-authorised incident to status while a TD spouse is not; EB-5 derivatives receive conditional residence alongside the investor; dependent children in the nonimmigrant categories lose status at twenty-one. None of these rules automatically describes the Gold Card framework.
Read the official page for what it says about a spouse, about children, about ages and about whether amounts are charged per person, and write down what you find in the government's own words. Where a household includes a child approaching twenty-one, give the age question specific attention, because in most immigration contexts that birthday changes eligibility. If the published material does not address a question your family needs answered, record it as unanswered and seek advice rather than filling the gap with an assumption.