how to find the gravitational force acting on an object?

If it is only gravitational force you want to know then you have all the data you need. Remember the force from gravity can always be found with formula:

Fg = g*m

Where Fg is the force of gravity, g is the gravitational acceleration and m is the mass. 

If it is the resulting forces on the object you need to find then it is another talk and you will need dynamic analysis