What is basically Gibb''s free energy change?Why it is required andHow is related to spontaneity of a chemical reaction ?

Gibb''s free energy is the useful work done by the system.....  G=H-TS.....If G is negative,the  rxn is spontaneous   &   if  G is positive , the rxn is non-spontaneous

The energy associated with a chemical reaction that can be used to do work.  The free energy of a system is the sum of its enthalpy (H) plus the product of the temperature (Kelvin) and the entropy (S) of the system...           
               DeltaG=Delta H - T Delta S
then a negative Delta G associated with a reaction indicates that it can happen spontaneously. This is consistent with the usual chemistry convention of treating work done by the system as negative work. Most common reactions can be assessed for spontaneity under standard conditions by looking up the associated thermodynamic quantities for each of the reactants and products. For non-standard conditions one can make use of the the expression for ΔG in terms of the other THERMODNAMIC POTENTIAL...

Energy available to do useful work is Gibb''s free energy. If change in Gibb''s free energy is negative, the reaction is spontaeous.