Gross National Product (GNP) measures the total economic output produced by a country's citizens and companies — anywhere in the world. If an Indian IT company earns revenue in the US, that counts toward India's GNP. If a US factory operates in India, its output counts toward US GNP, not India's.
GNP differs from GDP in one key way: GDP measures output within a country's borders (regardless of who produces it), while GNP measures output by a country's nationals (regardless of where they produce it). GNP = GDP + net income from abroad.
For most countries, GDP and GNP are similar. But for countries with large overseas workforces (like the Philippines, where remittances exceed $35 billion annually), GNP is significantly higher than GDP. Ireland's GNP is much lower than its GDP because multinational companies book massive profits there for tax purposes.