I guess you are trying to find out the foreign income in order to assess the export demand from those countries. If this is so, you should look at the trade % of USA with those countries. Then you can multiply that with the GNP of those countries to come up with a number that could be a representative for the foreign income which then determines the total export demand for the united states.

You got to think about the trade -weight here because even if a country has higher income, its importance to USA could be less if the demand for USA manufactured goods is less in that country.