GDP is the main indicator of national income as well as GNP. The reason why people go for GDP is that GDP is more often used and therefore easier to compare among the academics.

But again, the choice is largely depends on the authur' preference.

Before 1980,GNP was used more often.But as the globalization and international trade grows,GNP is more difficult to assess.So many people use GDP now.It is talked in Mankiw's macro economics.

懒的用英文表达了，GDP是一个容易得到的数据（相比GNP而言），可以反映本国的就业状况，不过有个缺陷，它其实并不能真实的反映一个国家的经济竞争力，真正能够反映本国民族产业发展的其实是GNP。对于美国而言，GDP和GNP 相差的并不多，而中国就不一样了。GNP远远小于GDP.