The relation of GDP and GNP is really a factor with taxation rule?

if the GDP of a nation costantantly fall from the last few year how can GNP help to improve GDP.or may be vice varsa.method of collecting the tax from sales and from income does really change to iprove GDP OR GNP.the taxation rule for the ^month in winter in all winter state (INCOME) if goes lower and the rest ^ month the same will it help to generate more liquid cash flow in the market to increse buying habit and maxmize the deprication through sale taxand gain a better GNP.in all the winter state of america..