What are the components of Gross National Product (GNP)? How does it understate

What are the components of Gross National Product (GNP)? How does it understate aggregate production in Third World countries where substantial economic production may be consumed directly and thus not reach markets where it can be measured? Finally is per capita income (PCI) a good measure of the level of living people enjoy or are the data skewed given substantial differences in the distribution of income especially in Third World countries? Construct a one-two paragraph posting in which you respond thoroughly supporting your answer with references.