how the GNP and GDP are used to determine the development of a country.

Your question has a little incorrectness about the cause and effect between the economic develpoment and data of GDP(GNP).The increase of data of the GDP is obviously an outcome of economic development.If you want know more on economics and GDP,it is advisable to get a macroeconomics textbook to go further.These is an extraordinarily popular textbook writen by Mankiw.You could see it everywhere,even downloading from some acadamic websites.Enjoy economis!