My question is why wouldn't we take the centre of mass of rod for the calculation of gravitation force because COM as also act as point mass and then Newton low of gravitation is valid

Rahul upadhyay Yes, I understand and I have given answer as per your question.
For the rod, the mass is distributed all over its entirety. And center of mass is not real thing. It is an imagination. Center of mass is an imaginary point where the whole mass of system is assumed. But for newton gravitation formula, it cannot be used because the formula is used for a point mass. And the uniform rod has a uniform mass distribution. And thus you cannot imagine or assume the whole mass to be concentrated at it's centre of mass while using the gravitational force formula.
Also remember the concept of moment of inertia in rotation, where you have derived the moment of inertia for a rod. At that time too, you didn't assumed the whole mass at center of instead during the derivation of moment of inertia from the mid point of rod, you have integrated it from -l/2 to +l/2.