If an object has a mass if 20kg what is the force of gravity acting on it on earth

That force is what most people would call the object's "weight".

Wherever the object is, its weight is

(mass) x (acceleration due to local gravity).

On Earth, the acceleration due to gravity is 9.8 m/s². (rounded)

The object's weight is 20 kg) x (9.8 m/s²) = 196 newtons.

(about 44.1 pounds)