While calcluating GNP, the overall income of a citizen working abroad is considered or only his/her remittances? Suppose a citizen of Country A is working in Country B with income X USD, and he sends Y USD as remittance to A, what is considered while calculating GNP? X or Y? (X is inclusive of Y)
GNI (Gross National Income) is the current description of what used to be called GNP.  It is GDP (Gross Domestic Product) plus net primary income from abroad (primary income receivable from the rest of the world less primary income payable to the rest of the world)

Assuming this individual is a long-term resident (1 year or more) in Country B, than none of this counts as Country A's GNI (Gross National Income is the current description of what used to be called GNP).  The earnings count in Country B's GDP (as part of the income measure) and GNI, while the remittance is part of Country A's secondary income from Country B and affects the Balance of Payments current accounts of the two countries.  See chapter 12 of the IMF Balance of Payments manual.  Something similar happens with the self-employed

If however this individual is a short-term resident (less than a year) or a cross-border worker, employee compensation counts as primary income and so is part of Country A's GNI though Country B's GDP (as part of the income measure).   Any goods or services this individual pays for in Country B is counted as a export from Country B to Country A. See chapter 11 of the IMF Balance of Payments manual. For short-term self-employed migrants, income counts as an export of services from Country B to Country A, and so counts in Country A's GDP and GNI, but again anything spent in Country B counts as trade the other way

Measuring these flows and making these distinctions are difficult for national statistical services, especially for migrant workers who do not know how long they will be staying in another country