Final answer:
The Income approach to measuring GDP involves summing all economic gains from labor, rentals, interest, and profits to provide a measure of a nation's economic activity.
Step-by-step explanation:
Measuring GDP as the total economic gains in labor, rentals, interest, and profits is called the Income approach. This method involves adding up all the incomes produced in a year to provide a measure of GDP. The income approach to GDP is equivalent to the expenditure and production approaches since it's based on the idea that all of the expenditures in an economy will eventually end up as someone's income. Thus, the total value of a nation's output is equal to the total value of a nation's income, making GDP and national income often used interchangeably.