Q.What is Net Factor Income from Abroad (NFIA)? How does it convert GDP into GNP?

NFIA is the net income a country's residents earn from the rest of the world, and it is exactly the adjustment that turns a domestic aggregate into a national one.

Net Factor Income from Abroad (NFIA) = (factor income earned by a country's normal residents from the rest of the world, e.g. wages of Indians working abroad, profits of Indian-owned firms operating overseas) − (factor income earned by non-residents from within the country, e.g. profits repatriated by a foreign company operating in India).

If NFIA is positive (residents earn more abroad than foreigners earn domestically), GNP is larger than GDP. If NFIA is negative — common in countries that host large amounts of foreign direct investment relative to how much their own residents invest abroad — GNP falls below GDP. For India, NFIA has generally stayed a small figure, reflecting sizeable inbound remittances from Indians working abroad set against profit outflows to foreign investors operating within India.

Assuming NFIA is always positive; forgetting the direction of subtraction (it is residents-earned-abroad MINUS non-residents-earned-domestically, not the reverse).