how can we say that gravity is avector quuantity as its so obvious that it acts in the downward direction. dont we just need to define the magnitude

First things first. Gravity is force!!. it is a force that acts on two objects by virtue of thier mass. This force is central...i.e it acts between centers of two objects...thus has direction.!

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Dear Rastogi,

                   Generally,gravity is nothing but the attraction between the centre of mass of two bodies where the magnitude is the centre of mass.Gravity is acting downwards in case of earth.Therefore,both magnitude and direction having a quantity is a vector.Therefore gravity is a vector. 

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